Fixes directory listing parsing for large directories

When listing a directory with several files, the information
will be available to the stream in multiple parts (by
multiple calls to the stream:handlEvent method). Sometimes this
information will be incomplete and won't be completely parsed.

If CFFTPCreateParsedResourceListing can't parse all the entries
in the listing, the remaining bytes in the buffer must be stored
to be parsed together when the stream has new bytes available.

This fixes #8