networkd: generate resolv.conf

networkd: generate resolv.conf

This adds support to generate a basic resolv.conf in /run/systemd/network.
This file will not take any effect unless a symlink is created from
/etc/resolv.conf.

Nameservers received over DHCP takes precedence over statically configured ones.

Note: /etc/resolv.conf is severely limited, so in the future we will likely
rather provide a much more powerfull nss plugin (or something to that effect),
but this should allow current users to function without any loss of
functionality.