Convert Settings to the new config system

This makes all config options self-documenting.

Unknown or unparseable config settings and --option flags now cause a
warning.

tests/shell.shebang.sh

@@ -1,4 +1,4 @@
 #! @ENV_PROG@ nix-shell
-#! nix-shell -I nixpkgs=shell.nix --option use-binary-caches false
+#! nix-shell -I nixpkgs=shell.nix --option build-use-substitutes false
 #! nix-shell --pure -i bash -p foo bar
 echo "$(foo) $(bar) $@"