Add file mode to the file-backup saved state blobs

This change puts the file's access mode into the saved-state blob used by the
file backup helpers.  The tests have been updated for the new blob content
format.

What this change *doesn't* do is actually backup/restore the file mode.  This
change is a prerequisite for that, but mode preservation in backup/restore will
require adding metadata to the backup data stream itself, so will be approached
a bit more carefully.

(Also fixed one outright bug in the test program: ReadEntityData() had been
changed to return a ssize_t union of either a byte-count or a negative number
indicating error, but the test program was still assuming that nonzero == error,
and was spuriously failing.)