* fhandler_tty.cc (fhandler_pty_master::setup): Add
	PIPE_REJECT_REMOTE_CLIENTS to pipe mode on systems supporting it.  Add
	FILE_FLAG_FIRST_PIPE_INSTANCE to pipe open mode.
	* pipe.cc (fhandler_pipe::create): Ditto.  Fix subsequent comment
	accordingly.
	* wincap.h (wincaps::has_pipe_reject_remote_clients): New element.
	* wincap.cc: Implement above element throughout.
	* winlean.h (PIPE_REJECT_REMOTE_CLIENTS): Temporarily define until
	Mingw64 headers define it.