Uppity: add command-line options to configure auth methods.

Now you can turn various authentication methods on and off, so that
the server won't even offer (say) k-i or publickey at all.

This subsumes the previous -allow-none-auth option; there's now a
general -{allow,deny}-auth=foo option schema, so -allow-auth=none is
the new spelling of -allow-none-auth. The former spelling is kept for
backwards compatibility, just in case.

ssh/server.h

@@ -21,7 +21,6 @@ struct SshServerConfig {
 
     bool stunt_pretend_to_accept_any_pubkey;
     bool stunt_open_unconditional_agent_socket;
-    bool stunt_allow_none_auth;
     bool stunt_allow_trivial_ki_auth;
     bool stunt_return_success_to_pubkey_offer;
 };