shared_mutex (__shared_mutex_pthread, [...]): New helper types implementing the shared mutex requirements.

	* include/std/shared_mutex (__shared_mutex_pthread,
	__shared_mutex_cv): New helper types implementing the shared mutex
	requirements.
	(shared_mutex): New type for C++17.
	(shared_timed_mutex): Derive from one of the new helper types.
	* testsuite/30_threads/shared_mutex/cons/1.cc: New.
	* testsuite/30_threads/shared_mutex/cons/assign_neg.cc: New.
	* testsuite/30_threads/shared_mutex/cons/copy_neg.cc: New.
	* testsuite/30_threads/shared_mutex/requirements/standard_layout.cc:
	New.
	* testsuite/30_threads/shared_mutex/try_lock/1.cc: New.
	* testsuite/30_threads/shared_mutex/try_lock/2.cc: New.

From-SVN: r224158