Add SO_DOMAIN constants to FreeBSD

[Thomasdezeeuw]

No description provided.

[rust-highfive]

r? @JohnTitor

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[Thomasdezeeuw]

Seems SO_PROTOCOL was already defined somewhere else, I've removed it.

[JohnTitor]

CI failed:

identifier 'SO_DOMAIN'
  cargo:warning=            static const int __test_const_SO_DOMAIN_val = SO_DOMAIN;
  cargo:warning=                                                          ^
  cargo:warning=1 error generated.
  exit code: 1

[Thomasdezeeuw]

It seems it isn't available on FreeBSD 11, I'll take a look at how to define it only on 12/13.

[Thomasdezeeuw]

@JohnTitor fixed it, it's now only defined on FreeBSD 12/13.

[JohnTitor]

Thanks!
@bors r+

[bors]

📌 Commit 01f7008 has been approved by JohnTitor

[bors]

⌛ Testing commit 01f7008 with merge 7e7452f...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: JohnTitor
Pushing 7e7452f to master...