libcommon: Continued cleaning up logical episode and/or map number usage

The goal being a straight map from URI to logical [episode &] map
number(s), allowing for dynamic reindexing of maps at runtime.

Todo: Presently, the "raw" logical numbers are still used by/for:
- G_NewGame(), G_DeferredNewGame() (need update API)
- Map specific playsim behavior (need generalize, expose via MapInfo)
- Save info header (switch to URI)
- Networked game state comms (need update protocol)