rename header->message, use correct option length
I should have used 'message' all along, since that's what the relevant RFC calls this. The 'header' is the first 236 bytes before the 'options'.
The min/max size for the message are added as macros, and the length of the options was adjusted based on further reading of RFC2131.
Lastly, this fixes an out-of-bounds access that was not caught by -Wall -Wextra, but only showed up with -Os