MFC r356138: Reimplement gvinum orphanization.

gvinum was the only GEOM class, using consumer nstart/nend fields. Making
it do its own accounting for orphanization purposes allows in perspective
to remove burden of that expensive for SMP accounting from GEOM.

Also the previous implementation spinned in a tight event loop, waiting
for all active BIOs to complete, while the new one knows exactly when it
is possible to close the consumer.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Prevent non-aligned reading from provider while tasting. Reject
providers with unsupported sectorsize.

Reported by: Joerg Wunsch
MFC after: 1 week

- Don't return EAGAIN from gv_unload(). It was used to work around the
deadlock fixed in r207671.
- Wait for worker process to exit at class unload. The worker process
was not guaranteed to exit before the linker unloaded the module.
- Use 0 as the worker process exit status instead of ENXIO and style
the NOTREACHED comment.

Reviewed by: lulf
X-MFC after: r207671

- Improve error message consistency and wording.

- Split the queue mutex into one for the event queue and one for the BIO queue,
as they do not really relate and to prepare for an additional queue to be
covered by the BIO queue mutex.
- Implement wrappers for fetching the next element from the event queue as well
as for putting a new element into the BIO queue.

- Add files that should have been added in r190507.