dhclient: add ability to ignore options in offers

A machine might exist on multiple networks, all of which offer, say, default
routes or name servers. There's no easy way to indicate in the config
that those options are only valid for a single interface.

Now, we can write:

interface "lan0" {
  request routers;
  require routers;
}
interface "lan1" {
  ignore routers;
}

And only take action on default routes offered on lan0.

Tested by: Jose Luis Duran <jlduran at gmail dot com>
MFC after: 2 months
Reviewed by: allanjude, imp
Sponsored by: Zenith Electronics LLC
Sponsored by: Klara, Inc.
Pull Request: #693