Add access to TOS(ECN bits) to the TCP syncache

[rscheff]

Summary

In order for the TCP Handshake to support ECN++,
and further ECN-related improvements, the ECN bits
need to be exposed to the TCP Syncache.

This change is a minimal modification to the
function headers, without any functional change.

The exception to this is the T4 tcp offload driver,
which also has a deep integration with TCP, and
the IP TOS byte (DiffServ + ECN bits) need to be
extracted there already.

A later patch will add the actual processing of
these bits in the syncache, in order to support
AccECN and/or SCE.