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]

misc.c

@@ -174,9 +174,9 @@ int bufchain_size(bufchain *ch)
     return ch->buffersize;
 }
 
-void bufchain_add(bufchain *ch, void *data, int len)
+void bufchain_add(bufchain *ch, const void *data, int len)
 {
-    char *buf = (char *)data;
+    const char *buf = (const char *)data;
 
     ch->buffersize += len;