openbsd_pkg: Add check_mode support.

The biggest change has been to package_latest since it was previously
just comparing version numbers before and after an upgrade had run.
We now parse the output from a dry run instead.

Thanks to Johan Belin for triggering the discussion :).
Merge pull request #3225 from eest/openbsd_pkg-check_mode

openbsd_pkg: Add check_mode support.