dhclient.conf.5: Add supersede interface-mtu to example

[emaste]

Summary

It seems to still be common for cable ISPs to return a bogus MTU (e.g.
576) via DHCP -- recently observed with Rogers business internet.

dhclient can be configured to ingore the server-provided MTU via the
`supersede interface-mtu` option.  This is already described in the man
page, but make it more accessible by including in the example config.

[markj]

sbin/dhclient/dhclient.conf.5

239

Maybe add a sentence here about when this might be useful?