Cygwin: posix timers: reimplement using OS timer

- Rename files timer.* to posix_timer.*.
- Reimplement using an OS timer rather than a handcrafted wait loop.
- Use a Slim R/W Lock for synchronization.
- Drop timer chaining.  It doesn't server a purpose since all timers
  are local only.
- Rename ttstart to itimer_tracker to better reflect its purpose.
  It's not the anchor for a timer chain anymore anyway.
- Drop fixup_timers_after_fork.  Everything is process-local, nothing
  gets inherited.
- Rename timer_tracker::disarm_event to disarm_overrun_event for
  better readability.

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

winsup/cygwin/cygheap_malloc.h

@@ -34,7 +34,8 @@ enum cygheap_types
   HEAP_2_DLL,
   HEAP_MMAP,
   HEAP_2_MAX = 200,
-  HEAP_3_FHANDLER
+  HEAP_3_FHANDLER,
+  HEAP_3_TIMER
 };
 
 extern "C" {