introduce an ECNCapablePacketConn interface to determine ECN support

[marten-seemann]

This PR implements solution 2 discussed in #2741 (comment).

TLDR: Currently we only enable ECN support if the user passes a naked *net.UDPConn into Dial and Listen, respectively. With this PR, we'll also enable ECN if the net.PacketConn is a wrapped net.UDPConn, as long as it exposes the required methods SyscallConn and ReadMsgUDP.

Note that this might contain an element of surprise for the user: To read packets from the connection, we won't use ReadFrom any longer, but ReadMsgUDP, so any wrapping of the ReadFrom will be bypassed. I added comments at multiple places to make users aware of this.

[codecov]

Codecov Report

Merging #2788 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2788   +/-   ##
=======================================
  Coverage   86.90%   86.90%           
=======================================
  Files         133      133           
  Lines       10102    10102           
=======================================
  Hits         8779     8779           
  Misses        990      990           
  Partials      333      333           

Impacted Files	Coverage Δ
client.go	80.33% <ø> (ø)
server.go	83.70% <ø> (ø)
conn.go	100.00% <100.00%> (ø)
conn_ecn.go	80.39% <100.00%> (ø)

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