(CFACT-41) Use resolv.conf for domain name fact resolution.

If we can't resolve the hostname, look for domain and search entires in
/etc/resolv.conf.

This allow us to conform to what facter is doing even when the hostname
is not resolvable.
I've updated the error case to use LOG_ERROR (shouldn't happen, so I think it's safe to properly treat that as an error) and the more common case of it not being resolved as LOG_INFO.