If protocol is "unix", use a Unix domain socket, not TCP

Fixes fallback to local connections from Xlib's XOpenDisplay(), which
will try with protocol "unix" if a hostname is specified and tcp fails
(as it usually will now that most OS'es ship with -nolisten tcp enabled).

Also fixes explicitly specifying DISPLAY as "unix/foo:0", which Xlib
previously accepted for Unix domain sockets.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>