Rename exit_client to exit_client_from, add exit_client to call
exit_client_from with from=me.

Modify exit_client_from to be able to propagate client QUITs without
having to inspect the 'from' parameter, leaving 'from' only used for
server quits.

Logically split the QS vs non-QS code apart by using sendto_server with
caps, which makes it much easier to read and understand.

Update almost all existing calls of exit_client to the newer version,
removing the 'from' parameter.