Work around a bug that was fixed after OpenBSD 5.0 - a request for
kern.hostname or kern.domainname with a nil value for oldp will result
in a length of zero being returned. If we hit this case use a length
of MAXHOSTNAMELEN (256).

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5408041