Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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.