Return early when remote DISPLAY variable set

When DISPLAY is set to a remote display, the remote display server
is down/unreachable and you su from user to root (or some other user),
this can take ~30 seconds+ to complete.

ps reveals ck-get-x11-server-pid is waiting on network connection to
the xserver that will eventually timeout.

Since it is impossible to get the process ID of the other end of a
TCP connection (manpages say Xlib uses TCP connections when
DISPLAY has a hostname: part) this patch skips getting the PID for
remote DISPLAYs.

References:
If the DISPLAY variable has a hostname component, e.g.
"remotehost:0.0", XOpenDisplay will use a TCP connection to it,
and the fd it returns later will be a TCP socket.
(http://tronche.com/gui/x/xlib/display/opening.html)

It is only possible to get the peer socket's PID for UNIX domain
sockets. (E.g. On linux, SO_PEERCRED is only usable for UNIX domain
sockets (i.e. PF_UNIX) )( http://linux.die.net/man/7/socket)

Bug report and patch provided by Joe Mullally.
https://bugs.launchpad.net/ubuntu/+source/consolekit/+bug/517323