The fast path in rtdm_mutex_timedlock() was terminally broken by the
not-so-recent addition of fast locks to RTDM-based kernel mutexes. As
a matter of fact, the xnthread_try_grab() helper suffered multiple
breakages, including the lack of update to the underlying fastlock
word to reflect a successful locking operation.

To this end, we introduce xnsynch_try_acquire() which properly
attempts to grab the requested mutex on behalf of the current thread,
and call it as a drop-in replacement for xnthread_try_grab().  Callers
fall back to xnsynch_acquire() on failure to put a fast grab on the
mutex.