MFC r269118: MFV r269010:

Import Illumos changes to address the following Illumos issues:
  4976 zfs should only avoid writing to a failing non-redundant
       top-level vdev
  4978 ztest fails in get_metaslab_refcount()
  4979 extend free space histogram to device and pool
  4980 metaslabs should have a fragmentation metric
  4981 remove fragmented ops vector from block allocator
  4982 space_map object should proactively upgrade when feature
       is enabled
  4984 device selection should use fragmentation metric
MFC r269138:

Add two sysctls for newly added tunables.