tcp: move ECN handling code to a common file

Reduce the burden to maintain correct and
extensible ECN related code across multiple
stacks and codepaths.

Formally no functional change.

Incidentially this establishes correct
ECN operation in one instance.

Reviewed By: rrs, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34162