MFC r324413, r324415

r324413:
Restore the ability to deregister an eventhandler from within the callback.

When the EVENTHANDLER(9) subsystem was created, it was a documented feature
that an eventhandler callback function could safely deregister itself. In
r200652 that feature was inadvertantly broken by adding drain-wait logic to
eventhandler_deregister(), so that it would be safe to unload a module upon
return from deregistering its event handlers.

There are now 145 callers of EVENTHANDLER_DEREGISTER(), and it's likely many
of them are depending on the drain-wait logic that has been in place for 8
years. So instead of creating a separate eventhandler_drain() and adding it
to some or all of those 145 call sites, this creates a separate
eventhandler_drain_nowait() function for the specific purpose of
deregistering a callback from within the running callback.

Differential Revision: https://reviews.freebsd.org/D12561

r324415:
Add eventhandler notifications for newbus device attach/detach.

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.

A couple salient comments from the review, they amount to some helpful
documentation about these events, but there's currently no good place for
such documentation...

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 arbitrarily. The handlers should be prepared
for such situations.

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.

Differential Revision: https://reviews.freebsd.org/D12557