Remove the shop hack.

You will be neither missed nor forgotten!

Tested save compat on old levels with one shop, with multiple shops,
and with new shops.
Refactor place_spec_shop a bit.
Don't call ball lightning creatures, either.
Replace a boolean field with a flag.
s/RETURN_FROM_/EXIT_/g
Arguably move recite text to data.

Unfortunately, there's still no simple way to add a new verse because
of the way that the hashing works; along the same lines, moving the
text to the database is non-trivial.