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.)
diff --git a/include/utils/BackupHelpers.h b/include/utils/BackupHelpers.h
index a21359f..759a0cc 100644
--- a/include/utils/BackupHelpers.h
+++ b/include/utils/BackupHelpers.h
@@ -43,6 +43,7 @@
 struct FileState {
     int modTime_sec;
     int modTime_nsec;
+    int mode;
     int size;
     int crc32;
     int nameLen;