Fix glitches with systemd sysusers and tmpfiles

Subject: [PATCH 1/2] Use descriptive file names for sysusers and tmpfiles configuration

These files were installed to:

${INSTALL_SYSTEMD_SYSUSERSDIR}/sysusers.conf

${INSTALL_SYSTEMD_TMPFILESDIR}/tmpfiles.conf

Instead rename the files to more descriptive file names 'mariadb.conf'.

Subject: [PATCH 2/2] Make systemd-tmpfiles create MYSQL_DATADIR

This is a no-op if the directory exists, but makes sure it is created by

systemd-tmpfiles with proper permissions otherwise.

This solves packaging issues when the user MYSQLD_USER is created by

systemd-sysusers and uid is not known in advance.