Load mods in "default" game alongside other mods

This allows us to use mods that are part of the default game. In other
words, all games can use the same default mod provided by the engine,
instead of needing their own copy.
Add default game mod loading to mods->list()
Reverse directory order in mods->list()

We want the files in higher-priority directories to be placed at the end
of the vector, so they get loaded after (and thus over) the previous
files.
Fix mod_dirs ordering and remove duplicate code
Merge pull request #644 from dorkster/default_game

Load mods in "default" game alongside other mods