Include comment header that describes overrides.

Unit description now includes @VERSION@.

After=syslog.target removed - redunant

Add --basedir=@prefix to prevent /root/.my.cnf lookups. This is
placed after $MYSQLD_OPTIONS in case a user sets a --{no,}default
type options which has to be first in the mysqld arguements.

Additional changes to multi instance (support-files/mariadb@.service.in):
* added @SYSTEMD_EXECSTARTPRE@ / @SYSTEMD_EXECSTARTPOST@
* removed mariadb@bootstrap reference as galera_new_cluster as
  it's a little too proment.
* use_galera_new_cluster.conf updated to override pre/post steps
  to ensure it has no side effects
Signed-off-by: Daniel Black <daniel@linux.vnet.ibm.com>