Refactor read_what() for clarity.

Basically, the dwarvish magazines ambiguate anything
else readable in the room (namely, the dark-room tablet
or the flowery message). The implementation is simplified
by the fact that the tablet and message are never present
together, and the endgame's inscribed oyster is *always*
the only readable object in the room.
Start work on Luckett & Pike's ADV440.
ADV440: FLY, RIDE, and SCORE.
ADV440: implement DROP and READ.