1) Only call crpytFlushData() if there *is* data.
2) Only call cryptFlushData() once per call to do_CryptFlush()
3) If cryptPushData() does a short write, flush immediately, regardless of
   the preferences
4) Pass the whole private data pointer to do_CryptFlush() so it can only
   call cryptFlushData() if there's unflushed data
5) Extend the private data structure to track unflushed data