ploop snapshot-list: fix usage

(1) All other tools print usage to stderr.
(2) All other tools say "Usage:" before synopsys.

Be the same.

Reference: ploop@971088
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
ploop snapshot-list usage: fit in 80 columns

Reference: ploop@971089
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
ploop snapshot-list usage: add -s

Reference: ploop@971090
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
ploop(8): document snapshot-list -s option

Reference: ploop@971091
Signed-off-by: Kir Kolyshkin <kir@openvz.org>