MFC r289362, r289445: 2605 want to resume interrupted zfs send

MFC r289362, r289445: 2605 want to resume interrupted zfs send

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed by: Xin Li <delphij@freebsd.org>
Reviewed by: Arne Jansen <sensille@gmx.net>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>

illumos/illumos-gate@9c3fd1216fa7fb02cfbc78a2518a686d54b48ab8

For more info, see:
 - slides http://www.slideshare.net/MatthewAhrens/openzfs-send-and-receive
 - video https://www.youtube.com/watch?v=iY44jPMvxog
 - manpage changes (for zfs resume -s and zfs send -t)
 - upcoming talk at the OpenZFS Developer Summit

The TL;DR is:
Use "zfs receive -s" to save the partially received state on failure.
On failure, get the receive token with "zfs get receive_resume_token <fs>"
Resume the send with "zfs send -t <token_value>"

Relnotes:       yes

git-svn-id: svn://svn.freebsd.org/base/stable/10@290756 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f