Made router discovery at least build and run. There are a few things
left to do (e.g. it doesn't yet run on systems with aliased addresses)
but this should work for simple configurations.

I don't plan to enable the rdisc directory in the sbin/ makefile until
I get feedback on this and add the missing features, so please, if you
have routers that perform router discovery, or if your FreeBSD box is
itself a router, give this a try.