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/dtable.cc

@@ -575,6 +575,9 @@ fh_alloc (path_conv& pc)
 	case FH_CYGDRIVE:
 	  fh = cnew (fhandler_cygdrive);
 	  break;
+	case FH_SIGNALFD:
+	  fh = cnew (fhandler_signalfd);
+	  break;
 	case FH_TTY:
 	  if (!pc.isopen ())
 	    {