Various ZED fixes

Various ZED fixes

* Teach ZED to handle spares usingi the configured ashift: if the zpool
   'ashift' property is set then ZED should use its value when kicking
   in a hotspare; with this change 512e disks can be used as spares
   for VDEVs that were created with ashift=9, even if ZFS natively
   detects them as 4K block devices.

 * Introduce an additional auto_spare test case which verifies that in
   the face of multiple device failures an appropiate number of spares
   are kicked in.

 * Fix zed_stop() in "libtest.shlib" which did not correctly wait the
   target pid.

 * Fix ZED crashing on startup caused by a race condition in libzfs
   when used in multi-threaded context.

 * Convert ZED over to using the tpool library which is already present
   in the Illumos FMA code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #2562
Closes #6858