Cygwin: timerfd: avoid a deadlock

Add a function timerfd_tracker::enter_critical_section_cancelable,
which is like enter_critical_section but honors a cancel event.  Call
this when a timer expires while the timerfd thread is in its inner
loop.  This avoids a deadlock if timerfd_tracker::dtor has entered its
critical section and is trying to cancel the thread.  See
https://cygwin.com/ml/cygwin/2019-06/msg00096.html.

winsup/cygwin/timerfd.h

@@ -86,6 +86,8 @@ class timerfd_tracker		/* cygheap! */
       return (WaitForSingleObject (_access_mtx, INFINITE) & ~WAIT_ABANDONED_0)
 	      == WAIT_OBJECT_0;
     }
+  /* A version that honors a cancel event, for use in thread_func. */
+  int enter_critical_section_cancelable ();
   void leave_critical_section ()
     {
       ReleaseMutex (_access_mtx);