Adding datadir argument to mysqld_safe command.

This addresses an issue where MariaDB would not load databases upon first starting after a system reboot.

My MariaDB installation went smoothly but I found that after restarting my system, the database server would not load any databases, only showing the defaults (mysql, performance_schema, etc). Issuing `mysql.server restart` always fixed it. Restarting the server using `launchctl` reliably reproduced the issue again, so I determined it had something to do with the plist configuration.

Adding the "datadir" command line argument to `mysqld_safe` fixes the problem.