Illumos 6815179, 6844191

Illumos 6815179, 6844191

6815179 zpool import with a large number of LUNs is too slow
6844191 zpool import, scanning of disks should be multi-threaded

References:
  https://github.com/illumos/illumos-gate/commit/4f67d75

Porting notes:
- This change was originally never ported to Linux due to it
  dependence on the thread pool interface.  This patch solves
  that issue by switching the code to use the existing taskq
  implementation which provides the same basic functionality.
  However, in order for this to work properly thread_init()
  and thread_fini() must be called around to taskq consumer
  to perform the needed thread initialization.

- The check_one_slice, nozpool_all_slices, and check_slices
  functions have been disabled for Linux.  They are difficult,
  but possible, to implement for Linux due to how partitions
  are get names.  Since this is only an optimization this code
  can be added at a latter date.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>