Fix culpable lack of generality in keyboard-interactive

Fix culpable lack of generality in keyboard-interactive
authentication: a k-i request packet can contain any number of auth
prompts (including zero!) and we must ask the user all of them and
send back a packet containing the same number of responses. FreeBSD
systems were sending a zero-prompts packet which was crashing us;
this now appears fixed (we correctly return a zero-responses packet)
but I haven't tested a multiple-prompts packet because I can't
immediately think of a server that generates them.

[originally from svn r1797]