MFV r302645: 6878 Add scrub completion info to "zpool history"

MFV r302645: 6878 Add scrub completion info to "zpool history"

illumos/illumos-gate@1825bc56e5a1f7ef6f0dc3137f3b35f5850c1100
https://github.com/illumos/illumos-gate/commit/1825bc56e5a1f7ef6f0dc3137f3b35f5850c1100

https://www.illumos.org/issues/6878
  Summary of changes:
      * Replace generic "scan done" message with "scan aborted, restarting",
        "scan cancelled", or "scan done"
      * Log number of errors using spa_get_errlog_size
      * Refactor scan restarting check into static function

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Nav Ravindranath <nav@delphix.com>
MFC after:	2 weeks