resolv-update script does not work with openresolv

If openresolv is installed instead of resolvconf, it overwrites resolv.conf after our script has written to it. We must find a way around this, or alternatively issue a warning if we detect (?) openresolv is installed.

https://launchpad.net/ubuntu/+source/openresolv http://packages.debian.org/cs/sid/openresolv

(from redmine: created on 2013-01-22, closed on 2016-08-25, relates #1257 (closed))