Cygwin: timers: implement timerfd

First cut of a timerfd implementation.

Still TODO:
- fork/exec semantics
- timerfd_settime TFD_TIMER_CANCEL_ON_SET flag
- ioctl(TFD_IOC_SET_TICKS)
- bug fixes

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

winsup/cygwin/devices.cc

@@ -123,6 +123,9 @@ const _device dev_pipew_storage =
 const _device dev_signalfd_storage =
   {"", {FH_SIGNALFD}, "", exists_internal};
 
+const _device dev_timerfd_storage =
+  {"", {FH_TIMERFD}, "", exists_internal};
+
 const _device dev_socket_storage =
   {"", {FH_SOCKET}, "", exists_internal};