OpenZFS 8959 - Add notifications when a scrub is paused or resumed

Authored by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed by: Alek Pinchuk <pinchuk.alek@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Gordon Ross <gwr@nexenta.com>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

Porting Notes:

• Brought #defines in eventdefs.h in line with ZFS on Linux format.
• Updated zfs-events.5 with the new events.

OpenZFS-issue: https://www.illumos.org/issues/8959
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c862b93eea
Closes #7049