Adding TF flags and SCF (syncache) flags for use with Accurate ECN.
Adding the TCP header Flag for the AE bit, which maps into the upper
portion of the reserved header flags range.
Also marking the last unused flag bit in the TF range, for future use.
These two bits in the TF appear to had prior (~FBSD 5.0, 2003?) use
around some other congestion control (AckCC) mechanism, but were
left unmarked and unclaimed.
This change has no functional changes to existing code.