dhclient(8): allow to supersede interface-mtu option

In some cases broken DHCP servers might send invalid MTU value, so allow to
use 'supersede' in dhclient.conf to override this. When superseded value is
0, MTU value is not updated at all.

PR: 206721
Submitted by: novel@
Reported by: <jimp AT pfsense.org>
MFC after: 37 minutes (if you care about 11, please MFC to 11.2)
Relnotes: yes (potentially surprising behavior change w/ broken dhcpd mtu)
Differential Revision: https://reviews.freebsd.org/D15484