Rework code to map channel IDs to hunt or puzzle objects from the database

Rework code to map channel IDs to hunt or puzzle objects from the database

The old functions were named things like channel_is_puzzle,
channel_is_hunt, and fund_hunt_for_channel. These were still expecting
the old multi-table database schema so they were broken, (as was all
functionality built on top of them such as the /state and /solved
commands).

The new functions are built on the all-one-table database schema, so
should actually work again. They also have a cleaner interface in that
they only rely only on channel ID (and never put any special stock in
the channel name). Instead of doing magic with the channel name, the
code looks up the hunt_id from the database entry for a puzzle as
needed.

The new functions are named puzzle_for_channel and hunt_for_channel
and are implemented on top of a common, internal function:
db_entry_for_channel.