This is in line with XDG Base Directory Specification, where ~/.config is supposed to be used when XDG_CONFIG_HOME is unset.
It also better matches the reality, where ~/.config/nix.conf does not seem to be used.
This removes the extra-substituters and extra-sandbox-paths settings
and instead makes every array setting extensible by setting
"extra-<name> = <value>" in the configuration file or passing
"--<name> <value>" on the command line.
