illumos 4970 through 4974 - extreme rewind enhancements

4970 need controls on i/o issued by zpool import -XF
4971 zpool import -T should accept hex values
4972 zpool import -T implies extreme rewind, and thus a scrub
4973 spa_load_retry retries the same txg
4974 spa_load_verify() reads all data twice
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>

References:
    https://www.illumos.org/issues/4970
    https://www.illumos.org/issues/4971
    https://www.illumos.org/issues/4972
    https://www.illumos.org/issues/4973
    https://www.illumos.org/issues/4974

Notes:
    This set of patches adds a set of tunable parameters for the "extreme
    rewind" mode of pool import which allows control over the implicit
    scrub performed during such an import.

Ported by: Tim Chase <tim@chase2k.com>