Cygwin: spawn: create and maintain winpid symlinks

- If the execve'ed process is a non-Cygwin process, we have to
  create the matching winpid symlink and remove the old one
  ourselves.

- If we spawn a child, the winpid symlink has to be maintained
  by the child process, otherwise it disappears if the parent
  process exits.

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

winsup/cygwin/pinfo.h

@@ -204,6 +204,7 @@ public:
   }
 #endif
   HANDLE shared_handle () {return h;}
+  HANDLE shared_winpid_handle () {return winpid_hdl;}
   void set_acl ();
   friend class _pinfo;
   friend class winpids;