Cygwin: signal: implement signalfd

First cut of a signalfd implementation.

Still TODO: Non-polling select.

This should mostly work as on Linux except for missing support
for some members of struct signalfd_siginfo, namely ssi_fd,
ssi_band (both SIGIO/SIGPOLL, not fully implemented) and ssi_trapno
(HW exception, required HW support).

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

winsup/cygwin/devices.cc

@@ -120,6 +120,9 @@ const _device dev_piper_storage =
 const _device dev_pipew_storage =
   {"", {FH_PIPEW}, "", exists_internal};
 
+const _device dev_signalfd_storage =
+  {"", {FH_SIGNALFD}, "", exists_internal};
+
 const _device dev_socket_storage =
   {"", {FH_SOCKET}, "", exists_internal};