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

Import adapted OpenSolaris' thread pool API implementation.

The thread pool is used by libzfs to implement parallel disk scanning.
Without this change our dummy wrapper made `zpool import ZZZ` command to
scan all disks sequentially from the single thread when searching for pools.
This change makes it use two threads per CPU, same as in OpenSolaris.

On system with 200 HDDs this change reduces ZFS pool import time from 35
to 22 seconds.

Finally... Import the latest open-source ZFS version - (SPA) 28.

Few new things available from now on:

- Data deduplication.
- Triple parity RAIDZ (RAIDZ3).
- zfs diff.
- zpool split.
- Snapshot holds.
- zpool import -F. Allows to rewind corrupted pool to earlier
transaction group.
- Possibility to import pool in read-only mode.

MFC after: 1 month