Make the game launchable on linux and use a better standard-like path for game data

As it is, the engine won't find the game data if the following is true:
1. The user is not in the directory containing the Doom 3 base folder
2. The binary is not installed in the same directory either (eg, in /usr/bin)
3. The user tries to simply launch the game by running the binary

Also, there's a semi-unrelated commit that changes the LINUX_DEFAULT_PATH. This can be rebased out if undesired, since it does use the default location that the binary doom3 installs to, but I figure this is nicer to be a set to a more typical path used by distros for installing proprietary/arch-independent game data, as distros like Debian, Fedora, and Arch do.