Move encryption initialization out of start_encryption and into accept_client.

This is consistent with its placement in the patch server and also just
makes more sense since this is where the client is initialized.
Add error check for allocated clients, add client d/c check, and handle return from client recv.
Implement skeleton of login_process_packet, adjust return type of receive_from_client and add docstring.
Remove unneeded fields from BANANA.
Remove old, unused connection handling code.
Fix comment, print statements for connection init and handling of send_welcome return value.

Also add implementation of send_packet (copied from patch_server) that
is intended to be used by the packet handling functions in login.cc.