Change the default DNS priority of VPNs to -50, to avoid leaking
queries out of full-tunnel VPNs.

This is a change in behavior. In particular:

 - when using dns=default (i.e. no split-dns) before this patch both
   VPN and the local name server were added (in this order) to
   resolv.conf; the result was that depending on resolv.conf options
   and resolver implementation, the name servers were tried in a
   certain manner which does not prevent DNS leaks.
   With this change, only the VPN name server is added to resolv.conf.

 - When using a split-dns plugin (systemd-resolved or dnsmasq), before
   this patch the full-tunnel VPN would get all queries except those
   ending in a local domain, that would instead be directed to the
   local server.
   After this patch, the VPN gets all queries.

To revert to the old behavior, set the DNS priority to 50 in the
connection profile.

(cherry picked from commit af13081b)