cygwin: simplify pthread timedwait handling

- Introduce inline helper pthread_convert_abstime.  It converts
  an absolute timespec to a Windows LARGE_INTEGER timestamp,
  depending on the used clock.

- Use this function from pthread_cond_timedwait and semaphore::timedwait

- Merge semaphore::_wait and semaphore::_timedwait into single _wait
  method, taking a LARGER_INTEGER timestamp.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/thread.h

@@ -693,11 +693,10 @@ public:
   }
 
 private:
-  int _wait ();
   void _post ();
   int _getvalue (int *sval);
   int _trywait ();
-  int _timedwait (const struct timespec *abstime);
+  int _wait (PLARGE_INTEGER timeout = NULL);
 
   void _fixup_before_fork ();
   void _fixup_after_fork ();