MDEV-33145 Add FLUSH GLOBAL STATUS

- FLUSH GLOBAL STATUS now resets most global_status_vars.
  At this stage, this is mainly to be used for testing.
- FLUSH SESSION STATUS added as an alias for FLUSH STATUS.
- FLUSH STATUS does not require any privilege (before required RELOAD).
- FLUSH GLOBAL STATUS requires RELOAD privilege.
- All global status reset moved to FLUSH GLOBAL STATUS.
- Replication semisync status variables are now reset by
  FLUSH GLOBAL STATUS.
- In test cases, the only changes are:
  - Replace FLUSH STATUS with FLUSH GLOBAL STATUS
  - Replace FLUSH STATUS with FLUSH STATUS; FLUSH GLOBAL STATUS.
    This was only done in a few tests where the test was using SHOW STATUS
    for both local and global variables.
- Uptime_since_flush_status is now always provided, independent if
  ENABLED_PROFILING is enabled when compiling MariaDB.
- @@global.Uptime_since_flush_status is reset on FLUSH GLOBAL STATUS
  and @@session.Uptime_since_flush_status is reset on FLUSH SESSION STATUS.
- When connected, @@session.Uptime_since_flush_status is set to 0.

mysql-test/suite/perfschema/t/show_coverage.test

@@ -93,7 +93,7 @@ SELECT variable_value FROM status_by_thread
   WHERE variable_name = "bytes_received" and thread_id = @thread_id INTO @bytes_before;
 --echo #
 --echo # Reset status counters
-FLUSH STATUS;
+FLUSH GLOBAL STATUS;
 --echo #
 --echo # Record bytes_received after reset
 SELECT variable_value FROM status_by_thread
@@ -170,7 +170,7 @@ SET long_query_time = @@global.long_query_time;
 --source include/assert.inc
 
 --echo #
-FLUSH STATUS;
+FLUSH GLOBAL STATUS;
 --echo #
 
 ## Global value requested from the default session after FLUSH must remain the same