Add SYSTEMD_READWRITEPATH-variable to mariadb{@,}.service.in to make sure that
if one is not building RPM or DEB packages then make sure there is ReadWritePaths
directive is defined in systemd service file.

This ensures that tar-ball installation has permissions to write database default
installation path (default: /usr/local/mysql/data) even if it's located
under /usr. Writing to that location is prevented by 'ProtectSystem=full'
systemd directive by default.

Prefixing the path with "-" in systemd causes there to not be an error if the
path doesn't exist. This may occur if the user has configured a datadir
elsewhere.

Reviewer: Daniel Black