interfaces: use /etc/network/interfaces instead of systemd

systemd-networkd is still a bit young. Let's switch to
/etc/network/interfaces. This leads to some other changes: IPv6 is now
setup through a DHCP hook. The firewall is not configuring IP addresses
anymore (except for IPv6).
interfaces: disable TSO on lan

This is the classic e1000e bug. Still here after many years...
interfaces: fix interface definition
interfaces: also is optional
interfaces: fix interface definition
interfaces: fix formatting of dhclient.conf
interfaces: source /etc/network/interfaces.d
interfaces: fix dnsmasq configuration
interfaces: more dnsmasq configuration fix
interfaces: fix /etc/network/interfaces syntax
interface: suffix udev rules by `.rules`
interfaces: move udev rules later

Otherwise, ID_PATH is not available
interfaces: disable use of persistent names
interfaces: fix dhclient configuration
interfaces: provide default DHCP options
interfaces: add blank space for dhclient configuration
interfaces: don't setup firewall using systemctl

It seems that systemd doesn't like that at all.
interfaces: don't let avahi try to detect anything about LL
interfaces: fix spelling of avahi-daemon
firewall: setup default NAT rule only inside main namespace