Cygwin: Implement pthread_tryjoin_np and pthread_timedjoin_np

- Move pthread_join to thread.cc to have all `join' calls in
  the same file (pthread_timedjoin_np needs pthread_convert_abstime
  which is static inline in thread.cc)
- Bump API version

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

winsup/cygwin/pthread.cc

@@ -41,12 +41,6 @@ pthread_exit (void *value_ptr)
   __builtin_unreachable ();	/* FIXME: don't know why this is necessary */
 }
 
-int
-pthread_join (pthread_t thread, void **return_val)
-{
-  return pthread::join (&thread, (void **) return_val);
-}
-
 int
 pthread_detach (pthread_t thread)
 {