Add eventhandler notifications for newbus device attach/detach.

[• ian]

Summary

The detach case is slightly complicated by the fact that some in-kernel consumers may want to know before a device detaches (so they can release related resources, stop using the device, etc), but the detach can fail. So there are pre- and post-detach notifications for those consumers who need to handle all cases.

[kib]

Note that in the current newbus locking model, DETACH_BEGIN and DETACH_COMPLETE/FAILED sequence of event handler invocation might interweave with other attach/detach events arbitrary. The handlers should be prepared for such situations.

[imp]

Is there a suitable man page to put the protocol on?

Otherwise lgtm.

[imp]

Also should note that detach may be called after the parent bus knows the hardware has left the building. In-kernel consumers have to be prepared to cope with this race...