Mark gozag shops as such in their spec

Does nothing, yet.
Add support for future changes to GOZAG_MAX_SHOPS

Move it from a define to a local, static function; check for each
index when considering initialization. (So we don't have to worry
about save compat.)
Guarantee a food shop in Gozag's Call Merchant

For non-foodless races.
Support gozag food shops for vp/gh

The former get blood; the latter get corpses.
Remove porridge/blood from potion petition

Now that food shops are guaranteed, it has no reason to exist,
and clutters up an otherwise tactical ability with a largely
strategic effect.

Potion base costs are now based on a weird and ugly heuristic.
This should probably be revised.
Remove bad potions from potion petition

The ability already has layers of cost: you need to spend gold to
use it (meaningful for gozagites), it costs a turn, it makes all
further petitions more expensive, and it requires you to worship
Gozag, a decision with significant tradeoffs of its own! There's
no design need to make it even *more* double-edged, and it made
the ability too weak in practice.
Update Gozag ability descriptions
Make !blood useless for non-vp

No longer needed, now that Gozag guarantees food shops to
carnivores. Aside from that, wholly redundant with every other
source of food.
Improve gozag shop quality

Give a 100% chance of GOOD_ITEMs, up from the base 25%.

Possibly this should be 50%, or perhaps there could be some
finer adjustment to item_level; this is a tentative start.
Remove porridge

Like blood for non-vp, this was no longer needed now that Gozag's
potion petition doesn't offer it. Again, it's redundant with other
food types, and pretty unexciting to boot. I mean... porridge?

I've replaced it with stacks of royal jelly in most cases, which
is a significant boost for carnivores (who got nothing at all from
porridge). If this is considered undesirable, they can be swapped
out for bread rations instead.