Adding max_blocksize setting

[r00ta]

In some environments clients are requesting a blocksize that is not compatible with the MTU of the network infrastructure. In TFTPD there is a max_blocksize option https://manpages.debian.org/testing/tftpd-hpa/tftpd.8.en.html

[--blocksize max-block-size, -B max-block-size](https://manpages.debian.org/testing/tftpd-hpa/tftpd.8.en.html#blocksize)
    Specifies the maximum permitted block size. The permitted range for this parameter is from 512 to 65464. Some embedded clients request large block sizes and yet do not handle fragmented packets correctly; for these clients, it is recommended to set this value to the smallest MTU on your network minus 32 bytes (20 bytes for IP, 8 for UDP, and 4 for TFTP; less if you use IP options on your network.) For example, on a standard Ethernet (MTU 1500) a value of 1468 is reasonable.

so this MP aims to add the same to this TFTP server.

Any feedback is very welcome!