Add MODs loading mechanism

The MOD makers can provide __init.nut and __initClient.nut to the
corresponding directory in mods subdirectory to register additional
MODs without changing codes in the main scripts.
Move loading scripts for gltestdll and surface into mods directory
Move gltestdll and surface modules to mods subdirectory

Also renamed gltestdll to vastspace, for it's a good chance to rename
a module.
surface: Move surface demo script to mod directory
vastspace: Move vastspace demo scripts to mod directory