kadm5: add chpass_with_key hook (#397)

kadm5: add chpass_with_key hook (#397)

Add a hook for changing a password with a key. This hook should be consolidated
into one shared with randkey and setkey, but for now I have continued to have
the hooks follow the kadm5 APIs themselves in both signature and quantity.

(This means the randkey one isn't actually very useful because it doesn't
provide the hook with the keys.)