Add channel program for property based snapshots

Channel programs that many users find useful should be included with zfs
in the /contrib directory. This is the first of these contributions. A
channel program to recursively take snapshots of datasets with the
property com.sun:auto-snapshot=true.

Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Clint Armstrong <clint@clintarmstrong.net>
Closes #8443
Closes #9050