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/ppl.h

@@ -114,7 +114,8 @@ PacketProtocolLayer *ssh2_userauth_new(
     bool show_banner, bool tryagent, bool notrivialauth,
     const char *default_username, bool change_username,
     bool try_ki_auth, bool try_gssapi_auth, bool try_gssapi_kex_auth,
-    bool gssapi_fwd, struct ssh_connection_shared_gss_state *shgss);
+    bool gssapi_fwd, struct ssh_connection_shared_gss_state *shgss,
+    const char *auth_plugin);
 PacketProtocolLayer *ssh2_connection_new(
     Ssh *ssh, ssh_sharing_state *connshare, bool is_simple,
     Conf *conf, const char *peer_verstring, bufchain *user_input,