SFTP: Replace incorrect comment about filesize. There is no 4 GiB limit.

* There is no 4 GiB limit. It has been removed by 2c80ac8acad6d4b0c486d4e5db63ac6e27b3e4ea
* The BigInteger comment is suboptimal. As we have seen, we can use floats for the time being.

I am not sure we can expect any problems with the actual statements below. Here is an alternative version which might have more desirable properties: `$attr['size'] = hexdec(bin2hex($this->_string_shift($response, 8)));`