Cygwin: FIFO: avoid hang after exec

Define fhandler:fifo::fixup_after_exec, which sets listen_client_thr
and lct_termination_evt to NULL.  This forces the listen_client thread
to restart on the first attempt to read after an exec.  Previously the
exec'd process could hang in fhandler_fifo::raw_read.

winsup/cygwin/fhandler_fifo.cc

@@ -942,6 +942,15 @@ fhandler_fifo::fixup_after_fork (HANDLE parent)
   fifo_client_unlock ();
 }
 
+void
+fhandler_fifo::fixup_after_exec ()
+{
+  fhandler_base::fixup_after_exec ();
+  listen_client_thr = NULL;
+  lct_termination_evt = NULL;
+  fifo_client_unlock ();
+}
+
 void
 fhandler_fifo::set_close_on_exec (bool val)
 {