backport: T4815: Fix various name server config issues

This is a backport of https://github.com/vyos/vyos-1x/pull/1656.

Note I also changed ip-down.script.tmpl to not wait for `systemctl
stop dhcp6c@$iface.service`, because that command is slow and pppd will
kill the ip-down script if it times out.

I didn't see ip-down.script.tmpl or its equivalent in the 1.4 branch.
Not sure if there is another mechanism to handle that functionality or
it is missed.