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.h

@@ -1301,6 +1301,7 @@ public:
   ssize_t __reg3 raw_write (const void *ptr, size_t ulen);
   bool arm (HANDLE h);
   void fixup_after_fork (HANDLE);
+  void fixup_after_exec ();
   int __reg2 fstatvfs (struct statvfs *buf);
   void clear_readahead ()
   {