wrong/inconsistent behaviour of global(-only) variables in session_track_system_variables

According to the documentation (https://mariadb.com/docs/server/server-management/variables-and-modes/server-system-variables#session_track_system_variables):

session_track_system_variables is a "Comma-separated list of session system variables for which to track changes"

In reality:

• global non-plugin variables are permitted in the list
• global plugin variables are removed from the list (again this is masked by MDEV-39207)
• value changes in global variables do not cause OK packet notification

To be consistent, global non-plugin variables should be removed from the list too.

With this test at the current MDEV-39207 patch:

enable_session_track_info;

set session session_track_system_variables='character_set_connection,max_connections';

select @@session.session_track_system_variables; # both are in the list

set character_set_connection=cp1251; # session track info printed

set global max_connections=100; # no session track info printed

set session session_track_system_variables='character_set_connection,myisam_block_size';

select @@session.session_track_system_variables; # only character_set_connection

set global myisam_use_mmap=1; # no session track info printed

set session session_track_system_variables=default;

set global max_connections=default,myisam_use_mmap=default;

Note that the MDEV-31609 implementation includes values of all variables (i.e. including global non-plugin ones) in the list in the initial OK packet, and the current MDEV-38179 patch continues with this convention. If there are no global variables in the list, neither needs to be adjusted but the MDEV-38179 patch logic can be simplified a little bit as either both plugin and global variable locks are needed or neither lock is.