shared_mutex (shared_timed_mutex): Add comments to explain the logic in the non-pthread_rwlock_t version.

	* include/std/shared_mutex (shared_timed_mutex): Add comments to
	explain the logic in the non-pthread_rwlock_t version.
	(_Mutex): Remove redundant type.
	(_M_n_readers): Rename to _S_max_readers.
	(_M_write_entered, _M_readers): New convenience functions.
	(lock, lock_shared, try_lock_shared, unlock_shared): Use convenience
	functions. Use predicates with condition variables. Simplify bitwise
	operations.
	(try_lock_for, try_shared_lock_for): Convert duration to time_point
	and call try_lock_until or try_shared_lock_until respectively.
	(try_lock_until, try_shared_lock_until): Wait on the condition
	variables until the specified time passes.
	(unlock): Add Debug Mode assertion.
	(unlock_shared): Add Debug Mode assertion.
	* testsuite/30_threads/shared_timed_mutex/try_lock/3.cc: New.

From-SVN: r221970