Illumos 4958, 5164, 5165

4958 zdb trips assert on pools with ashift >= 0xe
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>
    
References:
  https://www.illumos.org/issues/4958
  https://github.com/illumos/illumos-gate/commit/2a104a5
    
Porting notes:
 Keep the ZIO_FLAG_FASTWRITE define, because that is used in Linux but not in *BSD.

5164 space_map_max_blksz causes panic, does not work
5165 zdb fails assertion when run on pool with recently-enabled spacemap_histogram feature
Reviewed by: George Wilson george.wilson@delphix.com
Reviewed by: Christopher Siden christopher.siden@delphix.com

References:
  https://www.illumos.org/issues/5164
  https://www.illumos.org/issues/5165
  https://github.com/illumos/illumos-gate/commit/b1be289

Ported by: Turbo Fredriksson <turbo@bayour.com>