ZKAP requirement is miscalculated for shares larger than 2 GiB

Note that "shares" are storage for immutable files (storage for mutable files is in "slots"). This issue applies to shares.

Number of ZKAPs required for some amount of storage is based on that amount. The server-side code that determines the current size of a share is probably incorrect for files larger than 2 GiB. It reads size data from bytes 4-7 of the share file. However, the maximum size these bytes can represent is 2 GiB. When the share is larger, the real size has to be computed from the size of the container (the file) itself by subtracting some bookkeeping overhead from the total size.