OpenZFS 6051 - lzc_receive: allow the caller to read the begin record

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6051
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/620f322