OpenZFS 8264 - want support for promoting datasets in libzfs_core

OpenZFS 8264 - want support for promoting datasets in libzfs_core

Authored by: Andrew Stormont <astormont@racktopsystems.com>
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>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/8264
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/a4b8c9a
Closes #6254