MFV r268714:

MFV r268714:

Improve extreme rewind import.

When doing an "extreme rewind" import ("zpool import -XF"), we attempt
to verify all data in the pool, essentially scrubbing the entire pool.
The problem is that spa_load_verify_cb() issues an unbounded number of
concurrent scrub i/os.  This can lead to all of memory being used for
these zio's, wedging the system. Like normal scrub, we need to put a
cap on the number of outstanding i/os, and have the traverse thread
block when we reach this cap.

For this purpose the cap can be very large (10,000) to optimize the
elevator algorithm.  Three kernel tunables have been added:

	vfs.zfs.spa_load_verify_maxinflight
	vfs.zfs.spa_load_verify_metadata
	vfs.zfs.spa_load_verify_data

The latter two tunables controls whether metadata and/or user data
when doing extreme rewind.

Make 'zpool import -T' imply scrub.

Make zpool import -T <txg> accept hexadecimal values for the txg when
prefixed with 0x.

Skip txg's for which there is no uberblock when doing extreme rewind.

Skip reading all user data twice by skipping prefetches when doing
extreme rewinds as we do not access via the ARC.

Illumos issues:
  4970 need controls on i/o issued by zpool import -XF
  4971 zpool import -T should accept hex values
  4972 zpool import -T implies extreme rewind, and thus a scrub
  4973 spa_load_retry retries the same txg
  4974 spa_load_verify() reads all data twice

MFC after:	2 weeks