vncserver: Display check should confirm UNIX domain socket is still valid

vncserver: Display check should confirm UNIX domain socket is still valid

If the X server is not shut down cleanly, it can leave UNIX domain
sockets around that "vncserver" has previously mis-identified as
evidence that the display number is still in use.

Instead of checking for existence of /tmp/.X11-unix/X<n>, the code
will now attempt to connect to the socket to confirm that there is a
server process listening on this UNIX domain socket. This will
eliminate false positives in the case the UNIX domain socket still
exists but is not associated with a listening Xorg server. The Xorg
server does not have a problem with fixing this file when it next
starts up. It is only important to avoid using the port if it is still
in use.