scripts/mysql_system_tables_fix.sql to use alter table add/drop.. if exists/if not exists

ALTER TABLE ADD key/column IF NOT EXIST and ALTER TABLE DROP key/column IF EXISTS have been around since 10.0.2

scripts/mysql_system_tables_fix.sql statements don't include the existence checks.

The upgrade script should use them to reduce the errors in the log file of sql_error_log-plugin and just generally using the features available of MariaDB server.