Resource System: Changed default savegame folder to /home/savegames

For consistency with the naming convention used for most other types
of resource, the new savegame system by default uses /home/savegames
as the root savegame folder, for all games. Within this folder are
subfolders for each game, named according to the identity key of the
game for which the .save packages are intended.

Previously a game specific /home subfolder was used for the old format
savegame files (e.g., Doom used /home/savegame/<game-id> while Hexen
used /home/hexndata/<game-id>).

Existing old savegames are automatically converted by the engine and
will be output to the new /home/savegames/<game-id> location.

Keeping the new .save packages separate from the old savegames should
help to keep things tidy.