if a given filesystem were to disappear (e.g. removable storage)
followed by another filesystem appearing at the same location,
this would not get noticed by up2k in a timely manner
fix this by discarding the mtab cache after `--mtab-age` seconds and
rebuild it from scratch, unless the previous values are definitely
correct (as indicated by identical output from `/bin/mount`)
probably reduces windows performance by an acceptable amount
f6e693f0f5