mysql service: Shutdown with normal systemd SIGTERM instead of mysqladmin

According to the MySQL manual, this is a perfectly legal way of
shutting down the server. The shutdown logs also looks fine:

systemd[1]: Stopping MySQL Server...
mysqld[5114]: 140319  8:36:12 [Note] /nix/store/sc26mz82k97mbpx3d1abzn3rrbd155ws-mariadb-10.0.8/bin/mysqld: Normal shutdown
mysqld[5114]: 140319  8:36:12 [Note] Event Scheduler: Purging the queue. 0 events
mysqld[5114]: 140319  8:36:12 [Note] InnoDB: FTS optimize thread exiting.
mysqld[5114]: 140319  8:36:12 [Note] InnoDB: Starting shutdown...
mysqld[5114]: 140319  8:36:14 [Note] InnoDB: Shutdown completed; log sequence number 1619078
mysqld[5114]: 140319  8:36:14 [Note] /nix/store/sc26mz82k97mbpx3d1abzn3rrbd155ws-mariadb-10.0.8/bin/mysqld: Shutdown complete
systemd[1]: Stopped MySQL Server.