criu --help: fit to 80 columns

From where I stand, any help output that exceeds 80 columns is hardly
readable, so let's try to fit into that.

Apparently in many cases we can do that by removing some extra info
which, I guess, belongs to documentation or wiki but not to --help
output which should be just a short reference.

For this reason, this patch also removes information about environmental
variable set for external scripts -- such documentation does not belong
here.

Please review carefully to make sure I haven't thrown the baby out
with the bathwater.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>