Replacing dhcpcd with systemd-networkd and systemd-resolved

configs/releng/airootfs/etc/udev/rules.d/81-dhcpcd.rules:
Removing the udev rule for starting dhcpcd@.service on interfaces of the
form eth* and en*.

configs/releng/airootfs/etc/systemd/network/{ethernet,wireless}.network:
Adding default DHCP configurations for en*, eth*, wlp* and wlan*
interfaces.

configs/releng/airootfs/etc/resolv.conf:
Adding a symlink from /run/systemd/resolve/stub-resolv.conf to
/etc/resolv.conf (in airootfs).

configs/releng/airootfs/root/customize_airootfs.sh:
Adding systemd-networkd and systemd-resolved to the list of services
that are being enabled during airootfs customization.