Implemented zpool scrub pause/resume

Currently, there is no way to pause a scrub. Pausing may
be useful when the pool is busy with other I/O to preserve
bandwidth.

This patch adds the ability to pause and resume scrubbing.
This is achieved by maintaining a persistent on-disk scrub state.
While the state is 'paused' we do not scrub any more blocks.
We do however perform regular scan housekeeping such as
freeing async destroyed and deadlist blocks while paused.

tests/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_002_pos.ksh

@@ -62,7 +62,7 @@ log_onexit cleanup
 
 log_assert "Verify scrub, scrub -p, and scrub -s show the right status."
 
-log_must zinject -d $DISK1 -D10:1 $TESTPOOL
+log_must zinject -d $DISK1 -D20:1 $TESTPOOL
 log_must zpool scrub $TESTPOOL
 log_must is_pool_scrubbing $TESTPOOL true
 log_must zpool scrub -p $TESTPOOL