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]

windows/winx11.c

@@ -0,0 +1,18 @@
+/*
+ * winx11.c: fetch local auth data for X forwarding.
+ */
+
+#include <ctype.h>
+#include <assert.h>
+#include <stdlib.h>
+
+#include "putty.h"
+#include "ssh.h"
+
+void platform_get_x11_auth(struct X11Display *disp, const Config *cfg)
+{
+    if (cfg->xauthfile.path[0])
+	x11_get_auth_from_authfile(disp, cfg->xauthfile.path);
+}
+
+const int platform_uses_x11_unix_by_default = FALSE;