Improve missing- or wrong-hostkey interaction.

Specifically, both those now have three more options: show (re-print
the key fingerprints), full (print key data blob in full), and save-as
(which takes an argument, saving the key in the hostkey database under
that lookup key).  This involved improving the internal
prompted_choice() API.  Another improvement is that when reprompting,
it notices changes to the known-hosts database, in case the list of
host/port pairs the key is known for changes (which it typically will
after, eg, save-as).