dhcp: S80dhcp-server: also source /etc/default/dhcpd.conf for INTERFACES

Similar to how we do it for dhcpd.service. The file should really have been
named /etc/default/dhcpd instead, but changing it now would break existing
systemd setups so leave it as it is.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>