Fix bug in mariadb-service-convert

If mariadb-service-convert is run and the user variable is unset then
this sets `User=` in `[Service]`, which then tries to run mariadb as
root, which in-turn fails. This only happens when mysqld_safe is missing
which is all the time now. So...

1. Don't set `User=` if there is no user variable.
2. User mariadbd-safe instead.

Also
* galera_recovery to use mariadbd
* mtr - wsrep use mariadb executables
* debian/mariadb-server.mariadb.init use mariadbd-safe
* debian/tests/smoke uses mariadb instead of mysql as client.

Co-Author: Daniel Black <daniel@mariadb.org>