Add a preference list for SSH-2 key exchange algorithms, on a new "Kex" panel

(which will gain more content anon).

Retire BUG_SSH2_DH_GEX and add a backwards-compatibility wart, since we never
did find a way of automatically detecting this alleged server bug, and in any
case there was only ever one report (<3D91F3B5.7030309@inwind.it>, FWIW).

Also generalise askcipher() to a new askalg() (thus touching all the
front-ends).

I've made some attempt to document what SSH key exchange is and why you care,
but it could use some review for clarity (and outright lies).

[originally from svn r5022]

mac/mac.c

@@ -1,4 +1,4 @@
-/* $Id: mac.c,v 1.59 2003/05/10 12:27:38 ben Exp $ */
+/* $Id$ */
 /*
  * Copyright (c) 1999, 2003 Ben Harris
  * All rights reserved.
@@ -704,7 +704,7 @@ void verify_ssh_host_key(void *frontend, char *host, int port, char *keytype,
     }
 }
 
-void askcipher(void *frontend, char *ciphername, int cs)
+void askalg(void *frontend, const char *algtype, const char *algname)
 {
 
 }