server/chat.c: Don't send chat to unoccupied slots
host: Tweak host & peer SIGINT handling

Peer procs will now catch SIGINT and terminate, so SIGCHLD is sent to
the host proc. Host SIGINT will no longer disconnect peers, and instead
will send SIGINT to all existing peer procs. This way we can see
successful disconnect messages from the host SIGCHLD handler.