Windows: Refactor cm_Unlock*() to avoid code duplication

Windows: Refactor cm_Unlock*() to avoid code duplication

cm_Unlock() and cm_UnlockByKey() duplicate a significant amount
of code.  Refactor it into a new static function, cm_IntUnlock()
which handles the process of downgrading or releasing a file
server lock depending upon the lock state of the cm_scache_t
object.

Change-Id: Ic5db7b3928fc0477f155183326321717ea04ace0
Reviewed-on: http://gerrit.openafs.org/4923
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>