Pre-refax of change-independent parts for 2677

[ethouris]

Changes:

1. In LiveCC, the header size is taken now from CUDT::maxPayloadSize() and the current MSS size, to stay in sync with the value possibly differently elaborated in the future.
2. MSG TRUNC flag is now traced in the debug logs, to detect any problems resulting in getting bigger packets than expected.
3. checkIPv4Mapped added a simple wrapper to simplify its use.
4. In the receiver buffer, the average payload size is used without the initial size value, instead it recognizes a trap 0 value as "no packet received so far".