dhclient: add support for interface-mtu (26)

Make dhclient set interface MTU if it was provided.

This version implements MTU setting in dhclient itself before it runs
dhclient-script.

PR: 206721
Submitted by: novel@
Reported by: Jarrod Petz <jlpetz at gmail.com>
Reviewed by: cem, allanjude
Differential Revision: https://reviews.freebsd.org/D5675