Update to r42978:

This chapter finishes up the DHCP section. It does the following:

• moves the bpf warning higher as it applies to both
• reduces some headings
• removes redundancy and does general tightening
• clarifies some sections
• modernizes the server config file and expands on the descriptions

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D6035