Handle more gracefully the possibility of a keyboard-interactive

Handle more gracefully the possibility of a keyboard-interactive
prompts packet containing no actual prompts (perhaps due to odd
server organisation, or perhaps so it can print a banner message and
do nothing else). Previously, the get_userpass_input functions
always returned failure when in '-batch' mode, even in this case
where no actual input would be required.

[originally from svn r8490]