add ECN support

[marten-seemann]

Fixes #3999.

[codecov]

Codecov Report

Merging #4059 (d6ac630) into master (abfe1ef) will increase coverage by 0.07%.
Report is 1 commits behind head on master.
The diff coverage is 85.82%.

@@            Coverage Diff             @@
##           master    #4059      +/-   ##
==========================================
+ Coverage   83.42%   83.49%   +0.07%     
==========================================
  Files         147      148       +1     
  Lines       14888    15220     +332     
==========================================
+ Hits        12419    12707     +288     
- Misses       1972     2010      +38     
- Partials      497      503       +6     

Files Changed	Coverage Δ
internal/ackhandler/ackhandler.go	0.00% <0.00%> (ø)
internal/ackhandler/received_packet_tracker.go	82.64% <ø> (-0.14%)	⬇️
packet_handler_map.go	84.52% <ø> (ø)
sys_conn_helper_linux.go	75.41% <ø> (ø)
transport.go	72.31% <0.00%> (ø)
sys_conn.go	74.58% <33.33%> (-2.62%)	⬇️
internal/ackhandler/sent_packet_handler.go	74.61% <57.14%> (-0.67%)	⬇️
logging/multiplex.go	87.50% <66.67%> (-2.50%)	⬇️
server.go	75.93% <66.67%> (ø)
send_conn.go	86.96% <80.00%> (-0.28%)	⬇️
... and 10 more

... and 1 file with indirect coverage changes

[sukunrt]

Have closely reviewed the ecn.go file. Will finish the review after a couple of hours.

[peterjeremy]

This is wrong: It should be 1 for FreeBSD. See https://cgit.freebsd.org/src/tree/sys/netinet/udp_usrreq.c#n1144
4 is the correct length for use with setsockopt(2) but not as a cmsg.

[marten-seemann]

@peterjeremy You're commenting on a PR that was merged a couple of weeks ago. Please don't do this!

If you think you found an issue, please just open a new issue and we can continue discussing there.