libzfs_sendrecv: Style pass on send_iterate_fs

libzfs_sendrecv: Style pass on send_iterate_fs

* Capitalize and punctuate complete sentences in comments.
* Separate out a group of locals to add a comment on their purpose.
* Remove unnecessary line wrapping.
* Make it clear that dds_origin is a string by using explicit character
  comparison to check for an empty string, rather than implictly
  treating it as a boolean.
* Reorganize manipulation of props and holds nvlists to improve
  clarity.
* There's no need to initialize the snapname buffer with zeros, we're
  immediately overwriting it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12967