homed: enable userdb too if homed is requested

homed: enable userdb too if homed is requested

Strictly speaking you can run homed without userdb. But it doesn't
really make much sense: they go hand in hand and implement the same
concepts, just for different sets of users. Let's hence disable both
automatically by default if homed is requested.

(We don't do the reverse: opting into userdbd shouldn't mean that you
are OK with homed.)

And of course, users can always deviate from our defaults easily, and
turn off userbd again right-away if they don't like it, and things will
generally work.