ctdb: add ctdb_rundir parameter and create on startup

By default, ctdbd puts a .socket_lock file in RUNDIR/ctdb. The directory
must exist prior to startup.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Merge pull request #447 from ddiss/ctdb_rundir

ctdb: add ctdb_rundir parameter and create on startup