Lower vpn MTU from 1426 to 1312 because of DS-Lite connections. See thread on...

Lower vpn MTU from 1426 to 1312 because of DS-Lite connections. See thread on developer mailinglist "Änderung der MTU für DS-Lite Anschlüsse"

Signed-off-by: Clemens John <clemens.john@floh1111.de>