Much more work on Blksize support

tftpd.php:

* Fixed OACK packet diagram
* Defined TFTP_SEGSIZE in an if, after reading request for options
* Extended tftpd_recv_request to read options, specifically blksize and octets
* Checked taht $octets value is sane, and if it is assign TFTP_SEGSIZE from $octets if the request has options set, otherwise set to 512
* In tftpd_send_nak, add error 8, when Option negotiation fails

tftpd-config.php:

* Don't use 512, use TFTP_SEGSIZE instead

tftpd-file.php:

* Don't use 512, use TFTP_SEGSIZE instead