Common mods support

Implement "common mods", includeable from
  {$user,$share}/games/common/$modname
by using the game.conf setting
  common_mods = $modname,$modname2,...
Update README.txt to instruct to get minetest/common too
Update buildwin32.sh to get minetest/common
lua_api.txt: Document paths, games and common mod loading
Support game-specific minetest.conf
CMakeLists.txt: Add games/common to be installed when available
Add singlenode mapgen; generates solely the node 'mapgen_singlenode', defaults to air