pgsql: Refactor some duplicated code in lock.c: create UnGrantLock(),

Log Message:
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Refactor some duplicated code in lock.c: create UnGrantLock(), move code
from LockRelease() and LockReleaseAll() into it. From Heikki Linnakangas.

Modified Files:
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pgsql/src/backend/storage/lmgr:
lock.c (r1.145 -> r1.146)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c.diff?r1=1.145&r2=1.146)