- Remove blank setting values when setting has a group
 - Pair setting values with groups in file when possible
 - Preserve user-set whitespace in setting objects
 - Delete setting value when setting NoiseParams group
 - Delete overwritten groups outside of lock

Add format example to minetest.conf.example
Add Settings::setU16()
Throw exception on attempted access of NULL settings groups

Note: this does not fix the warnings generated by clang

See: https://github.com/minetest/minetest/issues/1790

Broken by 6bc4cad0, e.g.
  minetest --worldname world
would be parsed as
  minetest --worldname --worldname

This also cleans up settings a bit