snapshot_destroy: interpret snapshot ALL to mean destroy all

since snapshots are create as 'snap0', 'snap1', etc, this should not
be confusing.

Also add destroy to the manpage, as it was previously undocumented.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
snapshot tests: use snapshot_destroy(c, "ALL") to clean up

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>