Simplify piety for follower kills

It now follows the same rules as direct kills.

The old system was quite complex, in principle encouraged tedious
behaviour ('last-hitting'), and, most importantly, served no real
design purpose.

This is a buff to all gods who care about kills, especially those
(like Vehumet) who previously didn't accept follower kills at all.
If it's excessive, we can tone down piety gains or increase decay.

EXCEPTION: Ru still doesn't care about follower kills for purposes
of offering sacrifices; this is completely invisible to the player
(and was before) but is still a bug from my perspective. TODO.
Simplify
Fix yet another missing space issue

It will never be ended.......