ltq-vdsl-app: disconnect when service is stopped

ltq-vdsl-app: disconnect when service is stopped

Stop the connection when the control daemon is terminated. The code is
a modified version of the termination routine in version 4.23.1 of the
daemon (which doesn't support VR9 modems anymore).

This could also be implemented by calling the acos and acs commands via
dsl_cpe_pipe.sh in the init script. However, doing it in the daemon
itself has the advantage of also working if it is terminated in another
way (for example during sysupgrade).

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
(cherry picked from commit 1daaef31b35201a9c15a2084e25dac41b48c1867)