Allow zfs send to exclude datasets

Add support for a -exclude/-X option to zfs send to allow dataset
hierarchies to be excluded.

Snapshots can be excluded using a channel program; however,
this can result in failures with 'zfs send -R'; this option allows
them to be excluded. Fortunately, this required a change only to
cmd/zfs/zfs_main.c, using the already-existing callback argument
to zfs_send() that is currently unused.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Christian Schwarz <christian.schwarz@nutanix.com>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Sean Eric Fagan <kithrup@mac.com>
Signed-off-by: Sean Eric Fagan <kithrup@mac.com>
Closes #13158