Expand ECN support

[Ralith]

libc's CMSG macros got built out to many more platforms. There may remain some obscure platforms where this will still fail to build, but we can account for them as they show up in practice.

Note that some bugs remain in the new libc bindings, so we may want to defer merging this until rust-lang/libc#1235 shows up in a release.

[djc]

Yeah, waiting for the next release sounds like a good idea.

[codecov]

Codecov Report

Merging #222 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   74.82%   74.84%   +0.01%     
==========================================
  Files          24       24              
  Lines        6150     6150              
==========================================
+ Hits         4602     4603       +1     
+ Misses       1548     1547       -1

Impacted Files	Coverage Δ
quinn/src/platform/unix.rs	65.04% <ø> (ø)
quinn-proto/src/packet.rs	81.22% <0%> (+0.19%)	⬆️

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@92deb61). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #222   +/-   ##
=========================================
  Coverage          ?   74.82%           
=========================================
  Files             ?       24           
  Lines             ?     6150           
  Branches          ?        0           
=========================================
  Hits              ?     4602           
  Misses            ?     1548           
  Partials          ?        0

Impacted Files	Coverage Δ
quinn/src/platform/unix.rs	65.04% <100%> (ø)

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[Ralith]

I think this is ready for re-review and merge. If the macos inconsistencies are representative, there'll probably be additional tweaks necessary for other platforms, but only minor ones, and their necessity will be immediately obvious when socket configuration fails.