Replace outdated ISC DHCP 3.0 with version 3.1 from the ports collection.
The extension .sh from the startup script was removed as well, see
the entry of 20060818 in /usr/ports/UPDATING for details.

PR: docs/143120
Submitted by: Denny Lin (dennylin93 at cnmc32 dot hs dot ntnu dot edu dot tw)
Discussed with: jkois