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.

[originally from svn r8305]

putty.h

@@ -572,6 +572,7 @@ struct config_tag {
     int x11_forward;
     char x11_display[128];
     int x11_auth;
+    Filename xauthfile;
     /* port forwarding */
     int lport_acceptall; /* accept conns from hosts other than localhost */
     int rport_acceptall; /* same for remote forwarded ports (SSH-2 only) */