Merge ZFS feature flags support and related bugfixes:

Merge ZFS feature flags support and related bugfixes:
236884, 237001, 237119, 237458, 237972, 238113, 238391, 238422, 238926,
238950, 238951, 239389, 239394, 239620, 239774, 239953, 239958, 239967,
239968, 240063, 240133, 240153, 240303, 240345, 240415, 240955, 241655,
243014, 243505, 243506

MFC r236884:
Introduce "feature flags" for ZFS pools (bump SPA version to 5000).
Add first feature "com.delphix:async_destroy" (asynchronous destroy
of ZFS datasets).
Implement features support in ZFS boot code.

Illumos revisions merged:
13700:2889e2596bd6
13701:1949b688d5fb
2619 asynchronous destruction of ZFS file systems
2747 SPA versioning with zfs feature flags

References:
  https://www.illumos.org/issues/2619
  https://www.illumos.org/issues/2747

MFC r237001:
Fix ZFS boot with pre-features pools (version <= 28) broken in r236884

MFC r237119 [1]:
Do not remount ZFS dataset if changing canmount property to "on" and
dataset is already mounted.

MFC r237458:
Import Illumos revision 13736:9f1d48e1681f
2901 ZFS receive fails for exabyte sparse files

References:
  https://www.illumos.org/issues/2901

MFC r237972:
Expose scrub and resilver tunables.
This allows the user to tune the priority trade-off between scrub/resilver
and other ZFS I/O.

MFC r238113 (pjd):
vdev_io_done stage is not used for ioctls.

MFC r238391:
Change behavior introduced in r237119 to vendor solution

References:
  https://www.illumos.org/issues/2883

MFC r238422:
Merge illumos commit 13749:df4cd82e2b60

1796 "ZFS HOLD" should not be used when doing "ZFS SEND" froma read-only pool
2871 support for __ZFS_POOL_RESTRICT used by ZFS test suite
2903 zfs destroy -d does not work
2957 zfs destroy -R/r sometimes fails when removing defer-destroyed snapshot

References:
  https://www.illumos.org/issues/1796
  https://www.illumos.org/issues/2871
  https://www.illumos.org/issues/2903
  https://www.illumos.org/issues/2957

MFC r238926:
Partial MFV (illumos-gate 13753:2aba784c276b)
2762 zpool command should have better support for feature flags

References:
  https://www.illumos.org/issues/2762

MFC r238950:
Fix reporting of root pool upgrade notice.

MFC r238951:
Fix wrong indent according to style(9)

MFC r239389:
Backport fix for vendor issue #3085
3085 zfs diff panics, then panics in a loop on booting

References:
  https://www.illumos.org/issues/3085

MFC r239394:
Update zfs(8) manpage with illumos version of "zfs diff"

Illumos issue:
  2399 zfs manual page does not document use of "zfs diff"

References:
  https://www.illumos.org/issues/2399

MFC r239620 [2]:
Merge recent vendor changes:
3086 unnecessarily setting DS_FLAG_INCONSISTENT on async destroyed datasets
3090 vdev_reopen() during reguid causes vdev to be treated as corrupt
3102 vdev_uberblock_load() and vdev_validate() may read the wrong label

Referenes:
  https://www.illumos.org/issues/3086
  https://www.illumos.org/issues/3090
  https://www.illumos.org/issues/3102

MFC r239774:
Merge recent vendor changes:
3100 zvol rename fails with EBUSY when dirty
3104 eliminate empty bpobjs
3120 zinject hangs in zfsdev_ioctl() due to uninitialized zc

References:
  https://www.illumos.org/issues/3100
  https://www.illumos.org/issues/3104
  https://www.illumos.org/issues/3120

MFC r239953 (joel):
Mdoc fixes.

MFC r239958 (joel):
Minor mdoc fixes.

MFC r239967 (joel):
Mdoc fixes.

MFC r239968 (joel):
Remove trailing whitespace.

MFC r240063 (gjb):
Add myself to copyright sections, per CDDL license.

MFC r240133:
Merge recent vendor changes and sync code:
1862 incremental zfs receive fails for sparse file > 8PB
3112 ztest does not honor ZFS_DEBUG
3122 zfs destroy filesystem should prefetch blocks
3129 'zpool reopen' restarts resilvers
3130 ztest failure: Assertion failed:
       0 == dmu_objset_destroy(name, B_FALSE) (0x0 == 0x10)

References:
  https://www.illumos.org/issues/1862
  https://www.illumos.org/issues/3112
  https://www.illumos.org/issues/3122
  https://www.illumos.org/issues/3129
  https://www.illumos.org/issues/3130

MFC r240153 (gjb) [3]:
Typo fix and minor word swap.

MFC r240303:
Add assfail() and assfail3() to the opensolaris module.
Remove obsoleted intermediate cddl/compat/opensolaris/sys/debug.h.

MFC r240345 (avg):
zfs: fix sa_modify_attrs handling of variable-sized attributes

- skip length_idx index for a replaced variable-sized attribute
- skip length_idx index for a removed variable-sized attribute
- also re-arranged code to make sure that length_idx is always
  incremented for variable-sized attributes
- additionally add an assertion that the number of actually produced
  attributes is the same as the expected number of resulting
  attributes

MFC r240415:
Merge recent zfs vendor changes, sync code and adjust userland DEBUG.

Illumos issued covered:
1884 Empty "used" field for zfs *space commands
3006 VERIFY[S,U,P] and ASSERT[S,U,P] frequently check if first argument
     is zero
3028 zfs {group,user}space -n prints (null) instead of numeric GID/UID
3048 zfs {user,group}space [-s|-S] is broken
3049 zfs {user,group}space -t doesn't really filter the results
3060 zfs {user,group}space -H output isn't tab-delimited
3061 zfs {user,group}space -o doesn't use specified fields order
3064 usr/src/cmd/zpool/zpool_main.c misspells "successful"
3093 zfs {user,group}space's -i is noop
3098 zfs userspace/groupspace fail without saying why when run as non-root

References:
  https://www.illumos.org/issues/ + [issue_id]

MFC r240955 (partial):
Merge recent vendor changes in ZFS.

Illumos issued covered:
3139 zdb dies when it tries to determine path of unlinked file
3189 kernel panic in ZFS test suite during hotspare_onoffline_004_neg
3208 moving zpool cross-endian results in incorrect user/group accounting

References:
  https://www.illumos.org/issues/ + [issue_id]

MFC r241655:
Add missing initialization for do_prefix.
Corrects porting error in r238391

Vendor issue and changeset reference:
2883 changing "canmount" property to "on" should not always remount dataset
https://www.illumos.org/issues/2883
Changeset 13743:95aba6e49b9f

MFC r243014:
Move zpool-features manual page from section 5 to section 7
and fix references

Reported by:	pluknet

MFC r243505:
Illumos 13886:e3261d03efbf

3349 zpool upgrade -V bumps the on disk version number, but leaves
     the in core version

References:
  https://www.illumos.org/issues/3349

MFC r243506:
zfs sha256 checksum is missing in zfs.8 manpage

PR:	kern/167905 [1], kern/170912 [2], kern/170914 [2], doc/171356 [3]

git-svn-id: svn://svn.freebsd.org/base/stable/8@243717 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f