MFC r323355: MFV r323107: 8414 Implemented zpool scrub pause/resume

illumos/illumos-gate@1702cce751c5cb7ead878d0205a6c90b027e3de8
https://github.com/illumos/illumos-gate/commit/1702cce751c5cb7ead878d0205a6c90b027e3de8

FreeBSD note: rather than merging the zpool.8 update I copied the zpool
scrub section from the illumos zpool.1m to FreeBSD zpool.8 almost
verbatim. Now that the illumos page uses the mdoc format, it was an
easier option. Perhaps the change is not in perfect compliance with the
FreeBSD style, but I think that it is acceptible.

https://www.illumos.org/issues/8414

This issue tracks the port of scrub pause from ZoL: https://github.com/zfsonlinux/zfs/pull/6167
Currently, there is no way to pause a scrub. Pausing may be useful when
the pool is busy with other I/O to preserve bandwidth.

Description

This patch adds the ability to pause and resume scrubbing.  This is achieved
by maintaining a persistent on-disk scrub state.  While the state is 'paused'
we do not scrub any more blocks.  We do however perform regular scan
housekeeping such as freeing async destroyed and deadlist blocks while paused.

Motivation and Context

Scrub pausing can be an I/O intensive operation and people have been asking
for the ability to pause a scrub for a while. This allows one to preserve scrub
progress while freeing up bandwidth for other I/O.

Reviewed by: George Melikov <mail@gmelikov.ru>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Alek Pinchuk <apinchuk@datto.com>