net/dhcpd: doesn't start without discouraged -c flag

Remove discouraged -c flag from rc-file.
Also use PREFIX in config file

PR: 222151
Submitted by: Nikola Kolev <koue@chaosophia.net>
Reported by: mwlucas@michaelwlucas.com
Approved by: Nikola Kolev <koue@chaosophia.net>
MFC after: 2018Q2