Remove "user mode" for mod installs

[ngld]

Currently mods can be installed in "user mode" or "dev mode". "user mode" attempts to be compatible with older launchers (wxLauncher, YAL, ...) while "dev mode" is more flexible since each package has its own folder.
This means Knossos doesn't have to rely on FSO for the load order and can directly control it through ordering in the -mod flag.
It also allows us to enable or disable packages whenever we launch FSO.

Ref: https://www.hard-light.net/forums/index.php?topic=94068.msg1907741#msg1907741