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.

windows/help.h

@@ -120,6 +120,7 @@ typedef const char *HelpCtx;
 #define WINHELP_CTX_ssh_no_trivial_userauth "config-ssh-notrivialauth"
 #define WINHELP_CTX_ssh_auth_banner "config-ssh-banner"
 #define WINHELP_CTX_ssh_auth_privkey "config-ssh-privkey"
+#define WINHELP_CTX_ssh_auth_plugin "config-ssh-authplugin"
 #define WINHELP_CTX_ssh_auth_cert "config-ssh-cert"
 #define WINHELP_CTX_ssh_auth_agentfwd "config-ssh-agentfwd"
 #define WINHELP_CTX_ssh_auth_changeuser "config-ssh-changeuser"