Use minetest.conf for settings

[Poikilos]

Now config.lua uses minetest.conf settings for variables that would normally be configurable, instead of containing only hard-coded settings. The settingtypes.txt file included with this pull request describes the settings following Minetest standards.

[Sbgodin]

https://github.com/mt-mods/travelnet which is an updated fork of this repo would welcome your patch. As it enables to have one instance of the mod installed system-wide for a world-specific configuration.