siproxd: Service should restart when wan (outbound) interface goes up/down

[schuettecarsten]

I am using the stun plugin. On startup, siproxd tries to resolve the stun server address, this fails because my PPP connection takes some seconds to come up. When then outboung interface gets ready, siproxd should be restarted as other services also do.

Possible solution: https://github.com/openwrt/telephony/blob/master/net/rtpproxy/files/rtpproxy.hotplug

Notify maintainer: @guidosarducci