tcp: add TH_AE capabilities to ppp and pf

Add support for the AE Flag in the TCP header to pf and ppp.
Commonalize to the use of "E"(ECE), "W"(CWR) and "e"(AE)
for the TCP header flags, in line with tcpdump.

Reviewers: kp, cc, tuexen, cy, transport!
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D47106