send/recv: open up additional stream feature flags

The docs for drr_versioninfo have marked the top 32 bits as "reserved"
since its introduction (illumos/illumos-gate@9e69d7d). There's no
indication of why they're reserved, so it seems uncontroversial to make
a lot more flags available.

I'm keeping the top eight reserved, and explicitly calling them out as
such, so we can extend the header further in the future if we run out of
flags or want to do some kind of change that isn't about feature flags.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #15454