WL#4403: deprecate @log and @slow_log_queries variables

Affects: Server-5.1   —   Status: Complete   —   Priority: Medium

"log" and "log_slow_queries" command-line options and server variables duplicate
the functionality of the "general_log" and "slow_query_log" variables. They
should be deprecated. A warning should be added in 5.1, removal is scheduled for 7.0

Additionally, general_log_file and slow_query_log_file only exist as server
variables, not at options - one is supposed to use old --log and
--log_slow_queries to set file names. It is confusing, we should create
appropriate command-line options with matching names for them.

Notice that general_log_file and slow_query_log_file should only set the log
file name, without activating the logging (the desired behavior).
To mirror these variables correctly, we must also implement "general_log" and
"slow_query_log" as options. These two options will start the logging without
affecting the file names.