fs: fix --bwlimit timetables written out of order using the wrong limit

Until the first time slot of the week, the timetable used its last entry
as the limit carried over from the week before. That is only the latest
slot of the week when the entries are in order, so a timetable written
weekend first, like "Sat-00:00,off Mon-00:00,1M", limited Sunday to 1M
instead of leaving it unlimited.

Carry over the latest time slot of the week instead.
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