Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r258715: opensolaris compat: add taskq_wait emulation

MFC r258630: 734 taskq_dispatch_prealloc() desired

MFC r258628: opensolaris taskq: some cosmetic changes

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Don't access task structure once we call task function.
The task structure might be no longer available.
This also allows to eliminates the need for two tasks in the zio structure.

Submitted by: anonymous
MFC after: 2 weeks

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

Pass a format string to panic() and to taskqueue_start_threads().

Found with: clang

Add task structure to zio and use it instead of allocating one.
This eliminates the only place where we can sleep when calling zio_interrupt().
As a side-effect this can actually improve performance a little as we
allocate one less thing for every I/O.

Prodded by: kib
MFC after: 1 week

Partially MFp4 #176265 by pjd@:

- Properly initialize and destroy system_taskq.
- Add a dummy implementation of taskq_create_proc().

Note: We do not currently use system_taskq in ZFS so this is mostly a
no-op at this time. Proper system_taskq initialization is required
by newer ZFS code.

Ok'ed by: pjd
MFC after: 2 weeks

Remove OpenSolaris taskq port (it performs very poorly in our kernel) and
replace it with wrappers around our taskqueue(9).
To make it possible implement taskqueue_member() function which returns 1
if the given thread was created by the given taskqueue.

Approved by: re (kib)