In order to make "freeze" attacks against the update logic harder let's
add the ability to encode a "Best Before" date into SHA256SUMS directory
listings: if the current time is already beyond that time, we'll ignore
the SHA256SUMS as "stale" and fail the upgrade. Or in other words: the
freeze attack will now result in a client-side error eventually, instead
of success state.
The best before data is encoded in an optional pseudo-file listed in SHA256SUMS:
any file named BEST-BEFORE-YYYY-MM-DD.
Lennart Poettering
d0badc0a61