All ECN handling code in a common source file

[rscheff]

Summary

In order to keep the various stacks up to date
and not having to manually adjust ECN related
changes simultaneously at up to three different
places, extract the common code into dedicated
functions and refer to them instead.

[rscheff]

• clean commit against main, for ease of review

[pauamma_gundo.com]

No manual page change. Will review if and when one comes.

[rscheff]

• add ECN handling in the 3rd output path.

[rscheff]

sys/netinet/tcp_stacks/rack.c

16008

This is the section which was missing, and may end up sending a packet with unexpected IP ECN / TCP ECN bits from the last cached copy.

@rrs : If this is only ever called while TCPS_ESTABLISHED(), the special handling of SYN-RECEIVED could be removed, and the check if we are in >= SYN-RECEIVED above could be removed...

[rrs]

sys/netinet/tcp_stacks/rack.c

16008

I don't think it hurts to have the code, and I am not 100% sure that we wont get
called in SYN-RECV.. only 99.9 so leaving it is probably best.

[rscheff]

• restore diff after revert in main
• fix type of flag variable in rack.c

[rscheff]

sys/netinet/tcp_stacks/rack.c

15829

this was throwing compiler warn_as_err locally - CI compile error not yet reproduced

[rscheff]

• move tcp_set_flag aft the added setting of the ECN flags
• remove #include from tcp_ecn.h
• shuffle #include around, so that tcp_ecn.h doesn't need any
• restore #include in tcp_ecn.h