Revamp of the local X11 connection code. We now parse X display

Revamp of the local X11 connection code. We now parse X display
strings more rigorously, and then we look up the local X authority
data in .Xauthority _ourself_ rather than delegating to an external
xauth program. This is (negligibly) more efficient on Unix, assuming
I haven't got it wrong in some subtle way, but its major benefit is
that we can now support X authority lookups on Windows as well
provided the user points us at an appropriate X authority file in
the standard format. A new Windows-specific config option has been
added for this purpose.

git-svn-id: svn://svn.tartarus.org/sgt/putty@8305 cda61777-01e9-0310-a592-d414129be87e