Cygwin: introduce fhandler_process_fd and add stat(2) handling

move special fd symlink code into own fhandler_process_fd class
to simplify further additions to /proc/PID/fd/DESCRIPTOR symlink
handling.

Add a method to handle stat(2) on such a proc fd symlink by handle.
This allows correct reply from stat(2) if the target file has been
deleted.  This eventually fixes `awk -f /dev/fd/3 3<<eof'.

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

winsup/cygwin/dtable.cc

@@ -552,9 +552,11 @@ fh_alloc (path_conv& pc)
 	  fh = cnew (fhandler_registry);
 	  break;
 	case FH_PROCESS:
-	case FH_PROCESSFD:
 	  fh = cnew (fhandler_process);
 	  break;
+	case FH_PROCESSFD:
+	  fh = cnew (fhandler_process_fd);
+	  break;
 	case FH_PROCNET:
 	  fh = cnew (fhandler_procnet);
 	  break;