MFC 248571,248976,249004,249042,249188,249195-249196,249206,249207,249319,
    249326,249356-249357

Merge libzfs_core and other ZFS bugfixes and improvements.

MFC r248571:
  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

MFC r248976:
  Call dmu_snapshot_list_next() in zvol.c with dsl_pool_config lock held

MFC r249004:
  Do not check against uninitialized rc and comment out vendor code

MFC r249042:
  Fix possible pool hold leak in dmu_send_impl()

  Illumos ZFS issues:
    3645 dmu_send_impl: possibilty of pool hold leak

MFC r249188:
  Import vendor change to reduce diff, no effect on FreeBSD.

  Illumos ZFS issues:
    3517 importing pool with autoreplace=on and "hole" vdevs crashes
	 syseventd

MFC r249195:
  Merge change from vendor to reduce diff only.
  ZFS dtrace probes are not supported on FreeBSD yet.

  Illumos ZFS issues:
    3598 want to dtrace when errors are generated in zfs

MFC r249196:
  Provide a fix for kernel panic if receiving recursive deduplicated
  streams. Problem reported to vendor.

  Illumos ZFS issues:
    3692 Panic on zfs receive of a recursive deduplicated stream

MFC r249206:
  Merge vendor change - modify time processing in deadman thread.

  Illumos ZFS issues:
    3618 ::zio dcmd does not show timestamp data

MFC r249207:
  Allow zdb to output a histogram of compressed block sizes.

  Illumos ZFS issues:
    3641 want a histogram of compressed block sizes

MFC r249319:
  ZFS expects a copyout of zfs_cmd_t on an ioctl error. Our sys_ioctl()
  doesn't copyout in this case.

  To solve this a new struct zfs_iocparm_t is introduced consisting of:
  - zfs_ioctl_version (future backwards compatibility purposes)
  - user space pointer to zfs_cmd_t (copyin and copyout)
  - size of zfs_cmd_t (verification purposes)

  The copyin and copyout of zfs_cmd_t is now done the illumos (vendor) way
  what makes porting of new changes easier and ensures correct behavior if
  returning an error.

MFC r249326:
  Cast (void *)(uintptr_t) on copyout and copyin of zfs_iocparm_t.zfs_cmd

MFC r249356:
  Merge bugfixes accepted and integrated by vendor. Underlying problems
  have been reported by us and fixed in r240942 and r249196.

  Illumos ZFS issues:
    3645 dmu_send_impl: possibilty of pool hold leak
    3692 Panic on zfs receive of a recursive deduplicated stream

MFC r249357:
  Fix libzfs to report error instead of returning zero if trying to hold or
  release a non-existing snapshot of a existing dataset. In recursive case
  error is reported if no snapshots with the requested name have been found.

  Illumos ZFS issues:
    3699 zfs hold or release of a non-existent snapshot does not output
	 error