Fix a bug in pgxc_advisory_lock

The function should succeed if either the lock can be acquired OR
the lock is already held by the same transaction.
This adds -C option to gtm_ctl.   This allows DBA to choose alternative
gtm.control file, needed to specify restoration point of GTM when
coordinators/datanodes are recovered to specific point.

	modified:   src/bin/gtm_ctl/gtm_ctl.c