sbus(4) on OpenBSD

The text e4b3a3c56d0d, as OpenBSD 5.4, OpenBSD 5.3, OpenBSD 5.2, OpenBSD 5.1, OpenBSD 5.0, OpenBSD 4.9, OpenBSD 4.8, OpenBSD 4.7 shipped it.

introduction to SBus bus support