Smallfix: client_id more sensible.

Used to give id '1' to nobody, and '2' to the first client. Now
nobody's client_id is 0, and the first client is '1', which is less
surprising.
Fix: server doesn't stop when a client disconnects.

It was a SIGPIPE thing: the broadcast broadcasted the message about the
client disconnecting, and failed to exclude the client itself, so we
were writing in a broken pipe.

It's handled more sanely now.

Players disconnecting while playing are still not handled properly.