Fix noop receive of raw send stream

Currently, the noop receive code fails to work with raw send streams
and resuming send streams. This happens because zfs_receive_impl()
reads the DRR_BEGIN payload without reading the payload itself.
Normally, the kernel expects to read this itself, but in this case
the recv_skip() code runs instead and it is not prepared to handle
the stream being left at any place other than the beginning of a
record.

This patch resolves this issue by manually reading the DRR_BEGIN
payload in the dry-run case. This patch also includes a number of
small fixups in this code path.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #9221
Closes #9173