Final fixes to keyboard-interactive so it now works with packets

Final fixes to keyboard-interactive so it now works with packets
containing more than one prompt instead of less than one, and also
correctly enables echo on prompts that the server requests it for.
In the process I've moved the whole username/password input routine
out into its own function, where it's called independently of which
SSH protocol we're using, so this should even have _saved_ code
size. Rock!

[originally from svn r1830]