Support SO_REUSEPORT on darwin/etc

[andrewa-stripe]

Support for SO_REUSEPORT on darwin now exists
https://github.com/stripe/veneur/blob/3cec7b5735e6ea6b498aeb40ed36fc75af0cf824/socket_linux.go#LL23C5-L24C46

golang/go#16075

[andrewa-stripe]

The code looks like it may natively support it (as in we don't have have special cases) but the docs & comments could be updated to match
See: https://github.com/search?q=repo%3Astripe%2Fveneur%20SO_REUSEPORT&type=code