XXX comment in the logic which sends failed password auth back to username

XXX comment in the logic which sends failed password auth back to username
prompt for keyboard-interactive. I suspect we should do the same with that
method (especially given the apparent number of systems that use it for
regular password auth), but in the absence of systems to test against I've
not actually made the change. (I'm worried that the `partial success' field
might not be set correctly in a multi-stage authentication, for instance.)

git-svn-id: http://svn.tartarus.org/sgt/putty@4850 cda61777-01e9-0310-a592-d414129be87e