Improve System::config_resolver efficency when using defined_only

[nickanderson]

When using defined_only option resolv.conf is edited once for each resolver index slist value on each activation.

[tzz]

I tried to do this but couldn't find a better solution that would do good differential editing. Maybe someone else wants to try it; this is not a big inefficiency because that file is tiny, but I agree it's not optimal.