: Fix Reset/Sync-Flood Bug

[DCCP]: Fix Reset/Sync-Flood Bug

This updates sequence number checking with regard to RFC 4340, 7.5.4. Missing in the code was an exception for sequence-invalid Reset packets, which get a Sync acknowledging GSR, instead of (as usual) P.seqno.

This can lead to an oscillating ping-pong flood of Reset packets.

In fact, it has been observed on the wire as follows:

1. client establishes connection to server;
2. before server can write to client, client crashes without notifying

    the server (NB: now no longer possible due to ABORT function);

3. server sends DCCP-Data packet (has no ackno);
4. client generates Reset "No Connection", seqno=0, increments seqno;
5. server replies with Sync, using ackno = P.seqno;
6. client generates Reset "No Connection" with seqno = ackno + 1;
7. goto (5).

The difference is that now in (5) the server uses GSR. This causes the Reset sent by the client in (6) to become sequence-valid, so that in (7) the vicious circle is broken; the Reset is then enqueued and causes the socket to enter TIMEWAIT state.