hmi-controller: Check authorization of protocol binding.

> ..and use 'hmictrl' here instead of 'data'. That way the reader sees
> what the userdata for the wlresource is without tracking it further.
> This function is missing authorization checks. Any client is allowed to
> bind to hmicontroller and use it to do whatever they want. You'd
> probably rather want access to hmicontroller to be restricted to the
> client you launch in launchhmiclientprocess().
> To implement the authorization check, you would save the wlclient
> pointer from westonclientlaunch() or westonclientstart(), and compare
> 'client' to the saved pointer here. If the pointers differ, the client
> attempting to bind to hmicontroller is not the one you launched, and so
> you can send a protocol error and just return.
> If you want to implement respawn for the controller client, hook into
> the wlclient destruction, not SIGCHLD. You should hook to the wl_client
> destruction anyway, so you know when to reset the saved client pointer
> to NULL.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>