implement truncating of BSSGP STATUS when exceeding the FR MTU
It might be that we receive a BVC-STATUS on the IP side with a UDP packet size that exceeds the FR MTU, at which point we need to truncate it (see "NOTE" in 48.018 10.4.14).
The same can also occur when generating a BVC-STATUS in response to a BVC messag received on a FR link. Let's assume we e.g. receive a UL-UNITDATA that's exactly the FR MTU of 1600 for a non-existant BVCI. Then the generated BSSGP STATUS must truncate the "PDU IN Error" IE accordingly.
To make all of this work, we have to "learn" the MTU from the underlying network device (we receive it together with the link status event messages but currently ignore it). Plus we then need to use this value when generating STATUS or even when passing on STATUS