The previous way to initialize ~/srv/mariadb relied on /bin/mv, so that
it's atomic, making [template-mysqld-wrapper] idempotent (think of a
crash in the middle mysql_install_db). However, mv only works if
~/srv/mariadb is a simple directory: it fails if it's a symlink,
a mount point, or a subvolume (btrfs).

When using extra storage devices for some data, one may prefer to only
put the data (e.g. ~/srv/mariadb) and not the whole slapos partition.
Then if we want a resilient ERP5 with catalog's datadir on a separate
storage device, the importer script must be able to reinitialize
~/srv/mariadb whatever its type.