illumos gate

Make guid and createtxg public zfs properties.
Document said changes.
Fix a display bug with createtxg

ZoL has the following commit message:

commit 305bc4b370b20de81eaf10a1cf724374258b74d1
Author: Christian Schwarz <me@cschwarz.com>
Date: Wed May 10 00:36:53 2017 +0200

Make createtxg and guid properties public

Document the existence of `createtxg` and `guid` native properties
    in man pages and zfs command output.

One of the great features of ZFS is incremental replication of
    snapshots, possibly between pools on different machines.

Shell scripts are commonly used to auomate this procedure. They have to
    find the most recent common snapshot between both sides and then
    perform incremental send & recv.
    Currently, scripts rely on the sorting order of `zfs list`, which
    defaults to `createtxg`, and the assumption that snapshot names on
    either side do not change.

By making `createtxg` and `guid` part of the public ZFS interface,
    scripts are enabled to use

a) `createtxg` to determine the logical & temporal order of snapshots
         (the creation property is not an equivalent substitute since
          multiple snapshots may be created within one second)
      b) `guid` to uniquely identify a snapshot, independent of its current
          display name

This has the potential of making scripts safer and correct.

Reviewed-by: George Melikov <mail@gmelikov.ru>
    Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
    Reviewed-by: DHE <git@dehacked.net>
    Reviewed-by: Richard Laager <rlaager@wiktel.com>
    Signed-off-by: Christian Schwarz <me@cschwarz.com>
    Closes #6102