8264 want support for promoting datasets in libzfs_core

8264 want support for promoting datasets in libzfs_core

illumos/illumos-gate@a4b8c9aa65a0a735aba318024a424a90d7b06c37
https://github.com/illumos/illumos-gate/commit/a4b8c9aa65a0a735aba318024a424a90d7b06c37

https://www.illumos.org/issues/8264
  Oddly there is a lzc_clone function, but no lzc_promote function.

Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@kebe.com>
Approved by: Dan McDonald <danmcd@kebe.com>
Author: Andrew Stormont <astormont@racktopsystems.com>