[php-src] Issue #9673: $length argument for fpassthru

Issue: https://github.com/php/php-src/issues/9673 Author: divinity76 ### Description would be nice to have a ?int $length=null for fpassthru. currently, implementing [HTTP range requests](https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests) seems to be a pain: readfile() isn't suitable because it has neither a $offset nor a $length argument, and fpassthru isn't suitable either because it doesn't have a $length argument, so you'll have to resort to a fread()+echo loop. It would be easier and faster to implement HTTP range support if fpassthru had a $length argument.