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.

src/nix/main.cc

@@ -42,7 +42,6 @@ void mainWrapped(int argc, char * * argv)
     NixArgs args;
 
     args.parseCmdline(argvToStrings(argc, argv));
-    settings.update();
 
     assert(args.command);