nix: Make all options available as flags

Thus, instead of ‘--option <name> <value>’, you can write ‘--<name>
<value>’. So

  --option http-connections 100

becomes

  --http-connections 100

Apart from brevity, the difference is that it's not an error to set a
non-existent option via --option, but unrecognized arguments are
fatal.

Boolean options have special treatment: they're mapped to the
argument-less flags ‘--<name>’ and ‘--no-<name>’. E.g.

  --option auto-optimise-store false

becomes

  --no-auto-optimise-store

src/nix/main.cc

@@ -22,6 +22,8 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
         });
 
         mkFlag(0, "version", "show version information", std::bind(printVersion, programName));
+
+        settings.convertToArgs(*this);
     }
 };