Implement filter API for streams.

Implement filter API for streams.

Filters can be stacked onto a stream; more details will follow in docs and
on php-dev.

Implement "string.rot13" filter

Allows the following script:

$fp = fopen("file.txt", "r");
stream_filter_prepend($fp, "string.rot13");

// File contents will be subject to a rot13 transformation before
// being output.
fpassthru($fp);
fclose($fp);