Update zfs_admin_snapshot value (disabled)

It's disabled by default, update code and tests to reflect
the documentation.

Minor cleanup in delegate_common.kshlib.

Reviewed-by: Gregor Kopka <gregor@kopka.net>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #7835
Closes #8045