MFC r292435, r292441:

MFC r292435, r292441:
  - Keep hosts.by{name,addr} IPv4 only.
  - Add comment how we handle hosts and ipnodes.
  - Generate ipnodes.by{addr,name} from /etc/hosts for
    compatibility with FreeBSD local name resolution.
    If /var/yp/ipnodes exists, we generate them from it
    for backward compatibility.

git-svn-id: svn://svn.freebsd.org/base/stable/9@292721 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f