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]

putty.h

@@ -691,4 +691,14 @@ extern int cmdline_tooltype;
 
 void cmdline_error(char *, ...);
 
+/*
+ * X11 auth mechanisms we know about.
+ */
+enum {
+    X11_NO_AUTH,
+    X11_MIT,                           /* MIT-MAGIC-COOKIE-1 */
+    X11_NAUTHS
+};
+extern const char *const x11_authnames[];  /* declared in x11fwd.c */
+
 #endif