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]

mac/mac.c

@@ -1,4 +1,4 @@
-/* $Id: mac.c,v 1.22 2003/01/09 22:39:47 ben Exp $ */
+/* $Id: mac.c,v 1.23 2003/01/10 18:33:35 simon Exp $ */
 /*
  * Copyright (c) 1999 Ben Harris
  * All rights reserved.
@@ -740,6 +740,12 @@ int platform_default_i(char *name, int def)
     return def;
 }
 
+void platform_get_x11_auth(char *display, int *proto,
+                           unsigned char *data, int *datalen)
+{
+    /* SGT: I have no idea whether Mac X servers need anything here. */
+}
+
 /*
  * Local Variables:
  * c-file-style: "simon"