Introduce framework for authenticating with the local X server.

Windows and Mac backends have acquired auth-finding functions which
do nothing; Unix backend has acquired one which actually works, so
Plink can now do X forwarding believably.
(This checkin stretches into some unlikely parts of the code because
there have been one or two knock-on effects involving `const'. Bah.)

[originally from svn r2536]

winmisc.c

@@ -0,0 +1,14 @@
+/*
+ * winmisc.c: miscellaneous Windows-specific things.
+ */
+
+#include <windows.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include "putty.h"
+
+void platform_get_x11_auth(char *display, int *proto,
+                           unsigned char *data, int *datalen)
+{
+    /* We don't support this at all under Windows. */
+}