resolved: when rereading /etc/resolv.conf, always start using first DNS server again

Previously we tried to stick to a DNS server as long as it is available.
When /etc/resolv.conf changed, and the old DNS server we used was still
in there we'd continue to use it, even if it was at the end of the list.

With this change we'll now always start with the first DNS server in the
list again.

Rationale: certain network managing implementations (notably
NetworkManager) when connected to a VPN place both the VPN DNS server as
well as the local DNS server in /etc/resolv.conf. If we used the local
one before we would thus continue to use the local one, making VPN names
unresolvable. NetworkManager really should be fixed to only place the
VPN DNS servers in the file, but with this commit things are at least
similarly bad as they used to be...