Fix off-by-one bug in find_outdated_snapshot().

Fix off-by-one bug in find_outdated_snapshot().

The man page sayeth:

	"dss removes any snapshots older than n times u",

where n is the number of unit intervals and u is the duration of
a unit interval. As intervals count from zero, this means that a
snapshot should be considered outdated if its interval number
is greater _or equal_ than n.

However, the current code only removes snapshots in intervals
strictly greater than n. Fix this bug and clarify the documentation.