WL#4903: Plug-in Service API: part 1

This is a subset of WL#3859 which provides the automatic versioning protection
for plugins that use server functionality. It allows server to create services
and plugins to use them. Automatic version checks and dynamic linking on load is
implemented. Manual load (using get_service) with version negotiation is not.
Reference counting is not implemented either - it's only needed when plugins
will *provide* services, not *use* them. Now all services are statically
compiled into the server. Thus no service needs to be "released", and releasing
is not implemented either.

Services will be declared in include/mysql/service_XXX.h files.
To avoid a long list of #include's in plugin.h (to keep plugin.h readable),
plugin.h will include services.h, which, in turn, will include all service_XXX.h
files. All service declarations will be controlled by abi_check rule
(indirectly, via plugin.h.pp).

Service versions will be in include/service_versions.h (this file shouldn't be
installed on a target system), as a list of defines - one per service. Symbols
with the versions that will be replaced by pointers to the real services (see
WL#3859) will be in a libservices.a, which will be in
libservices/. It's crucial to have one file per service there.

Services will be defined in sql/sql_plugin_services.h, which will be included
into sql/sql_plugin.cc

An example service will be implemented - "my_snprintf" service, which provides
my_snprintf and my_vsnprintf. daemon plugin will make use of it.