WL#11828: Convert syslog/Eventlog-related system-variables to component variables

When the new structured / pluggable error logging was developed, logging to 
unix's syslog / Window's EventLog was moved to a loadable component. As the 
component framework did not at that time support component variables (pluggable 
system variables), the system variables controlling the operation of 
syslogging/Eventlogging have remained in the server, alongside some of the 
logging functionality.

Now that component variables are available, the syslog/Eventlog variables should 
be updated to use them to achieve the following:

- less surprising behavior: currently, the component can be unloaded, but the 
system variables remain visible to the user. Assigning values to these variables 
is not possible in this state: the code deciding on the validity of submitted 
values has been unloaded and, with no one to approve the new value, we must 
reject it by default. (It's better to throw a visible error here than to let the 
user think they succeeded in setting a value that is invalid at worst, and 
without effect at best.)
This also led to SET PERSIST breaking in certain cases, as persisted values can 
not be restored when the component to approve them is not loaded.

- cleaner separation of concerns -- we should endeavour to have most of the code 
concerned with syslogging/eventlogging in the same place; we should not have half 
of it in the server and static, and the other half unloadable in a module, if we 
can avoid it.

- stream-lining of the code-base: Component variables were not available when the 
first logging components were written. In their stead, code to propagate changes 
in system variables to listeners in log-service components was built into the new 
logging engine. This code can be removed once its last user, the syslog/eventlog 
component, no longer needs it.

1 Functional Requirements

1.1 Pluggable system variables

The variables controlling syslogging/Event-logging should move from the server 
proper into the sys/eventlog component, and be namespaced accordingly.

This will also fix the problem of "log_sys_* global variables do not work with 
set global/persist".


2 Non-functional Requirements

2.1 Update log-service API

The structured logging engine features functionality intended to let loaded 
components have system-variables that somewhat duplicates functionality now 
added to the component framework.

FR-1.1.1 will remove the last user of error-logging's variable handling, as 
variable handling is changed over to the component framework's new API for this. 
This suggests the following steps to remove the now dead code and clean up the 
API:

2.1.1 Remove sysvar check/update forwarding from logging engine

2.1.2 Remove calls for sysvar check/update from log-service API

2.1.3 Remove handlers for sysvar check/update from log-services

2.1.4 Add function returning log-component characteristics to API

As part of updating the API, the a function returning the information about the 
following characteristics shall be added to provide the foundation for requested 
diagnostics and functionality:

a) Is it a source, a filter, or a sink?
Use case: this will let us throw warnings when trying to set suspicious or 
unsupported pipeline layouts (e.g. having a filter as the last element in the 
pipeline).

b) Is it read-only?
I.e., does it promise not to change the log-event?
Use case: lets us determine whether it's safe to e.g. run several writers in 
parallel.
(Writers guaranteeing this would also implicitly promise not to use the log-
event's default iter.)

c) Is it a singleton, or can it be opened multiple times?
Use case: complaining when a singleton, such as the syslog writer, is used more 
than once in the logging pipeline.


2.2 Move syslog/eventlog-related code from server into component where possible

This was the original design, which was deferred. Amend by moving
syslog/eventlog-related code from server into component where possible.

I-1  SQL syntax

NO CHANGE

I-2  Instrumentation

NO CHANGE

I-3  Error and Warnings

NO CHANGE

I-4  Install/Upgrade

Some system variable names are changed.
Configurations (my.cnf) that use the old variable names must be updated.
Due to implementation details of the interim variable handler, the settings 
could not be PERSISTed using the old names, so an upgrade of persisted values 
should not be necessary.

I-5  Commercial Plugins

NO CHANGE

I-6  Replication

NO CHANGE

I-7  Protocols

NO CHANGE

I-8  Security

NO CHANGE

I-9  Log files

NO CHANGE

I-10 MySQL clients

NO CHANGE

I-11 Auth plugins

NO CHANGE

I-12 Globalization

NO CHANGE

I-13 Configuration

Syslog/Eventlog-related system variables are renamed:
@@global.log_syslog_X becomes @@global.syseventlog.X.
The on/off toggle @@global.log_syslog is removed  entirely, as on/off is now 
expressed by the component's inclusion in, or exclusion from, the logging 
pipeline, @@global.log_error_services.
See below, Low-Level Design Specification, for a complete list.

I-14 File formats

NO CHANGE

I-15 Plugins/APIs

Error-logging component API:
- new function: log-components' characteristics (read-only, multi-open, 
source/sink/filter) can be queried
- removed functions: sys-var check/update functions are removed as their 
functionality is now covered by per-component system-variables. The 
syslog/Eventlog-writer was the last user of this interface.

I-16 Other programs

NO CHANGE

I-17 Storage engines

NO CHANGE

1     System variables

1.1   Move sys/Event-log variables into component namespace

These variables should move from the server-proper into the sys/eventlog 
component, and be namespaced accordingly. All "@@global.log_syslog_X"
shall be renamed to ""@@global.syseventlog.X"; the on/off toggle log_syslog
shall be removed entirely (as it is replaced by adding the component to or
removing it from @@global.log_error_services):

@@global.log_syslog_tag         -> @@global.syseventlog.tag
@@global.log_syslog_facility    -> @@global.syseventlog.facility
@@global.log_syslog_include_pid -> @@global.syseventlog.include_pid
@@global.log_syslog             -> removed


1.2   Remove --log-syslog=0 argument from mysql-test-run

As this system variable will be removed in 1.1.1.1, mtr should not set it.


2     log-service API

2.1   remove interim log-service variable-handling code

2.1.1 Remove LogVar class from sql/log.h

2.1.2 Remove LogVar class and log_broadcast_sys_var_set() from 
components/mysql_server/log_builtins.cc

2.1.3 Remove listeners/stubs from log-services

2.2   Add support for a function that may return the following log-component 
characteristics

log_service_imp::characteristics:

LOG_SERVICE_READ_ONLY = 1,   // service promises not to modify event

LOG_SERVICE_SINGLETON = 2,   // only one instance may be opened

LOG_SERVICE_SOURCE    = 32,  // service is a log-source (reserved)
LOG_SERVICE_FILTER    = 64,  // service is a log-filter
LOG_SERVICE_SINK      = 128  // service is a log-sink