openvpn plugin: Implement the “ImprovedNamingSchema” config option.

I'm attaching a new version that includes this config key. By default the old
naming schema is used, while for the single mode the new naming schema is
always used since there's no backward compatibility to be preserved. You can
use the previous file[1] for the 5.0 branch where only the new naming schema
will be used. A new man page text is still lacking, maybe I'll write it in the
next days.