reader: transform get_bidder into register_bidder

reader: transform get_bidder into register_bidder

There's a notable duplication across all the read bidder code.

Check the archive magic/state, set the bidder private data (to NULL in
all but one case), name and vtable.

Change the helper to do the actual registration, keeping things simpler
in the dozen+ filters. This also allows us to enforce the bidder ::bid
and ::init dispatch are non NULL. The final one ::free is optional.

NOTE: some of the bidders do _not_ set a name, which I suspect is a
pre-existing bug. I've left them as-is, but we might want to fix and
enforce that somehow.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>