Addition of flag bit definitions in preparation of AccECN

[rscheff]

Summary

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.

[rscheff]

• minimizing delta vs. HEAD (see D22428)

[rgrimes]

Something really strange here with this diff

[rscheff]

In D22429#490316, @rgrimes wrote:

Something really strange here with this diff

The raw diff should be ok though...

[rgrimes]

In D22429#490320, @rscheff_gmx.at wrote:

In D22429#490316, @rgrimes wrote:

Something really strange here with this diff

The raw diff should be ok though...

Yes, must be a display artifact in Phabricator.

[rscheff]

sys/netinet/tcp_var.h

330 ↗

(On Diff #64484)

These flags are also being used in other (uncommitted) patches.
Will move all ECN-related Flags to TF2 instead.

[rscheff]

• remove ACE flag from TF and keep flag bits reserved