stream-unix: Do not bind a name for client sockets.

There's no reason for a Unix domain client socket to bind a name.  I don't
know why we've always done that.  Stevens's "Unix Network Programming"
Unix domain socket client example doesn't do a bind.

Removes the 'unlink_path' parameter from new_fd_stream() since it is now
always passed as NULL.

Signed-off-by: Ben Pfaff <blp@nicira.com>