cygwin: simplify pthread timedwait handling

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>