Clean up the freedoom/freedm ebuilds

  - Removed `inherit games` line from both, it was only used to get a single
    variable (`GAMES_DATADIR`). If that pathname ever *becomes* variable then
    we're probably screwed anyway.

  - Changed install location from `/usr/share/games/doom-data` to
    `/usr/share/games/doom`; I can't find a justification for the former other
    than "historical baggage", and the latter is a built-in default in Odamex.

  - Made `pkg_postinst` just print the filenames outright instead of a vague
    warning about symlinks.