- consistently set options, searches, domains fields to %NULL,
  if there are no values.

- in nm_global_dns_config_update_checksum(), ensure that we uniquely
  hash values. E.g. a config with "searches[a], options=[b]" should
  hash differently from "searches=[ab], options=[]".

- in nm_global_dns_config_to_dbus(), reuse the sorted domain list.
  We already have it, and it guarantees a consistent ordering of
  fields.

- in global_dns_domain_from_dbus(), fix memleaks if D-Bus strdict
  contains duplicate entries.