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/fhandler.cc

@@ -1345,6 +1345,9 @@ fhandler_base::fstat (struct stat *buf)
     case FH_PIPER:
       buf->st_mode = S_IFIFO | S_IRUSR;
       break;
+    case FH_SIGNALFD:
+      buf->st_mode = S_IRUSR | S_IWUSR;
+      break;
     default:
       buf->st_mode = S_IFCHR | STD_RBITS | STD_WBITS | S_IWGRP | S_IWOTH;
       break;