Disable AP_REGISTER because of possible exploiting and uselessness

Any authenticated client (= correct username + password) can basically write "unlimited" text to the file "regform.txt" in the blakserv directory by using AP_REGISTER message.

This feature is not used by the current client and has security issues, someone might try to "DoS" the server harddisk with lots and big writes until it is full or fails.

This change does not fully remove the message from the protocol, but instead disable the processing of such messages on the server-side.