Add openbsd_pkg: package management for OpenBSD.

* Code influenced by the service module but without classes.
* Thanks to @dhozac for input and review.
* Thanks to @johanwiren for getting me to push this out the door.
Teach setup about OpenBSD package manager.

* We cannot depend on the path because FreeBSD for example uses the same path.
Merge pull request #2283 from eest/openbsd_pkg

Add openbsd_pkg: package management for OpenBSD.