zpool: Provide GUID to zpool-reguid(8) with -g (#16239)

This commit extends the zpool-reguid(8) command with a -g flag, which
allows the user to specify the GUID to set.

This change also adds some general tests for zpool-reguid(8).

Sponsored-by: Wasabi Technology, Inc.
Sponsored-by: Klara, Inc.

Signed-off-by: Mateusz Piotrowski <0mp@FreeBSD.org>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>