Support for 203 ("204") protocol

We would like to use this old open source bot on Uranium: https://gitlab.com/StormyRSC/sts203c

It could be useful for finding authenticity bugs.

It would also make it very easy to port OpenRSC to 201, which had many interesting clients too (although less notable open source ones).

Necessary changes:

• The way the "session ID" is sent to the client needs to be changed. Currently login attempts from a 203 client will time out, because the client sends a packet requesting the session ID, and the server cancels sending the session ID when it receives a packet.