Tighten FAT checks and fix off-by-one error in corner case.
- Only truncate out-of-range cluster pointers (1, or greater than NumClusters but smaller than CLUST_RSRVD), as the current cluster may contain some data. We can't fix reserved cluster pointers at this pass, because we do no know the potential cluster preceding it.
- Accept valid cluster for head bitmap. This is a no-op, and mainly to improve code readability, because the 1 is already handled in the previous else if block.
- truncate_at: absorbed into checkchain.
- checkchain: save the previous node we have traversed in case that we have a chain that ends with a special (>= CLUST_RSRVD) cluster, or is free. In these cases, we need to truncate at the cluster preceeding the current cluster, as the current cluster contains a marker instead of a next pointer and can not be changed to CLUST_EOF (the else case can happen if the user answered "no" at some point in readfat()).
- clearchain: correct the iterator for next cluster so that we dont stop after clearing the first cluster.
- checklost: If checkchain() thinks the chain have no cluster, it doesn't make sense to reconnect it, so don't bother asking.