Start dhcpcd/wpa_supplicant after systemd-udev-settle

This is necessary to prevent a race.  Udev 197 has a new naming scheme
for network devices, so it will rename (say) eth0 to eno0.  This fails
with "error changing net interface name eth0 to eno1: Device or
resource busy" if another process has opened the interface in the
meantime.
Add/fix systemd unit descriptions