Set hell beasts to the top of their HD, AC, EV (but not speed)

Hell beasts are weak for their demon tier. They're basically redundant
on nearly all fronts but Geryon. As such, I'm tweaking an odd and
pointless set-ups: they would previously have a random 4 to 7 HD,
which corresponded with 2 to 5 AC and 11 to 15 EV. Since they're
relatively weak, I'm just going to fix this bit of randomness directly,
giving all of them 7 HD / 5 AC / 15 EV. Still weak in plenty of other ways.

They're still random speeds, because random speed is significant and
obviously visible. Abominations, though not particularly amazing in
Abyss, have a revalent aspect of player Twisted Resurrection to them
that I'd rather not touch.
(Strictly speaking, the Horn of Geryon especially and a variety of
other greater demon summons are slightly buffed player-wise, but it's
in a marginal enough manner that I'm not concerned.)