syscall: hostname/domainname fix for openbsd
syscall: hostname/domainname fix for openbsd

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
http://codereview.appspot.com/5408041

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