- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFV r254421:

Illumos ZFS issues:
3996 want a libzfs_core API to rollback to latest snapshot

MFV r252215:

Restore a previous behavior before r251646, where when destructing
ZFS snapshot, the ioctl would return ENOENT when it hit any of
them in the errlist (the new behavior was only return ENOENT when
all returns error).

Illumos ZFS issues:
3829 fix for 3740 changed behavior of zfs destroy/hold/release ioctl

MFC after: 1 week

MFV r251644:

Poor ZFS send / receive performance due to snapshot
hold / release processing (by smh@)

Illumos ZFS issues:
3740 Poor ZFS send / receive performance due to snapshot
hold / release processing

MFC after: 2 weeks

Merge libzfs_core branch:
includes MFV 238590, 238592, 247580

MFV 238590, 238592:
In the first zfs ioctl restructuring phase, the libzfs_core library was
introduced. It is a new thin library that wraps around kernel ioctl's.
The idea is to provide a forward-compatible way of dealing with new
features. Arguments are passed in nvlists and not random zfs_cmd fields,
new-style ioctls are logged to pool history using a new method of
history logging.

http://blog.delphix.com/matt/2012/01/17/the-future-of-libzfs/

MFV 247580 [1]:
To address issues of several deadlocks and race conditions the locking
code around dsl_dataset was rewritten and the interface to synctasks
was changed.

User-Visible Changes:
"zfs snapshot" can create more arbitrary snapshots at once (atomically)
"zfs destroy" destroys multiple snapshots at once
"zfs recv" has improved performance

Backward Compatibility:
I have extended the compatibility layer to support full backward
compatibility by remapping or rewriting the responsible ioctl arguments.
Old utilities are fully supported by the new kernel module.

Forward Compatibility:
New utilities work with old kernels with the following restrictions:
- creating, destroying, holding and releasing of multiple snapshots
at once is not supported, this includes recursive (-r) commands

Illumos ZFS issues:
2882 implement libzfs_core
2900 "zfs snapshot" should be able to create multiple,
arbitrary snapshots at once
3464 zfs synctask code needs restructuring

References:
https://www.illumos.org/issues/2882
https://www.illumos.org/issues/2900
https://www.illumos.org/issues/3464 [1]

MFC after: 1 month
Sponsored by: Hybrid Logic Inc. [1]

MFV r247580:
Merge synctask code restructuring from vendor.

Modify forward and backward compatibility to support new change.

Illumos ZFS issues:
3464 zfs synctask code needs restructuring

Sponsored by: Hybrid Logic Ltd.

Move common zfs ioctl compatibility functions (userland) into libzfs_compat.c
Introduce additional constants for zfs ioctl versions

Fix working with zfs_ioctl_version in libzfs_compat.h and include mirror
lzc_ioctl_version in libzfs_core

Add forwards compatibility for libzfs_core
Unsupported: creation of multiple snapshots including "zfs snapshot -r"

Move libzfs compat functions to libzfs_compat.h
This header is used by both libzfs_core and libzfs libraries

WiP merge of libzfs_core (MFV r238590, r238592)
not yet working, ioctl handling needs to be changed

Update vendor-sys/illumos/dist to illumos-gate 13752:9f5f6c52ba19
(zfs part)

Obtained from: ssh://anonhg@hg.illumos.org/illumos-gate