* Introduce new "rc-manager=auto" setting and make it the default,
  unless a different default is chosen at compile time.
  This mode tries to detect "systemd-resolved", "resolvconf", and "netconfig"
  and chooses the mode that seems most suitable depending on build
  setting and runtime detection.
  "resolvconf" and "netconfig" are only considered iff NetworkManager
  was built with the respective options enabled.