openbsd_pkg: Handle another pkg_add gotcha

* Add '-m' to pkg_add incovation to get access to the "packagename-1.0: ok"
  message.
* Watch for that message if we are about to fail because of stderr in
  package_present().
openbsd_pkg: sync package_latest().

This diff syncs package_latest() with the changes to package_present().
I have not managed to figure out how to handle the cornercases where
stderr is set but the command has not failed, so leave a FIXME blob for
other adventurers.