New feature: k-i authentication helper plugins.

In recent months I've had two requests from different people to build
support into PuTTY for automatically handling complicated third-party
auth protocols layered on top of keyboard-interactive - the kind of
thing where you're asked to enter some auth response, and you have to
refer to some external source like a web server to find out what the
right response _is_, which is a pain to do by hand, so you'd prefer it
to be automated in the SSH client.

That seems like a reasonable thing for an end user to want, but I
didn't think it was a good idea to build support for specific
protocols of that kind directly into PuTTY, where there would no doubt
be an ever-lengthening list, and maintenance needed on all of them.

So instead, in collaboration with one of my correspondents, I've
designed and implemented a protocol to be spoken between PuTTY and a
plugin running as a subprocess. The plugin can opt to handle the
keyboard-interactive authentication loop on behalf of the user, in
which case PuTTY passes on all the INFO_REQUEST packets to it, and
lets it make up responses. It can also ask questions of the user if
necessary.

The protocol spec is provided in a documentation appendix. The entire
configuration for the end user consists of providing a full command
line to use as the subprocess.

In the contrib directory I've provided an example plugin written in
Python. It gives a set of fixed responses suitable for getting through
Uppity's made-up k-i system, because that was a reasonable thing I
already had lying around to test against. But it also provides example
code that someone else could pick up and insert their own live
response-provider into the middle of, assuming they were happy with it
being in Python.

ssh/ssh.c

@@ -267,14 +267,14 @@ static void ssh_got_ssh_version(struct ssh_version_receiver *rcv,
                     conf_get_bool(ssh->conf, CONF_try_gssapi_auth),
                     conf_get_bool(ssh->conf, CONF_try_gssapi_kex),
                     conf_get_bool(ssh->conf, CONF_gssapifwd),
-                    &ssh->gss_state
+                    &ssh->gss_state,
 #else
                     false,
                     false,
                     false,
-                    NULL
+                    NULL,
 #endif
-                    );
+                    conf_get_str(ssh->conf, CONF_auth_plugin));
                 ssh_connect_ppl(ssh, userauth_layer);
                 transport_child_layer = userauth_layer;