Native networking should use SO_REUSEADDR where appropriate

[alexcrichton]

Not doing so is causing spurious failures on the bots I believe. This is already done by libuv for us, and I think that libnative should do the same thing in order to keep the two in sync.

Last I looked, I think there's also some business with SO_REUSEPORT for udp sockets. Regardless, I think that we should mirror what libuv is doing for both tcp and udp.

[bnoordhuis]

Apropos UDP, the SO_REUSEADDR flag lets the process steal the port from another process. Libuv does it for (bug-for-bug) backwards compatibility with node.js v0.4 but it's something of a questionable practice. Looking back at it, we should have made it opt-in.

If you're going to be feature compatible with libuv, then this comment may be useful.

[alexcrichton]

Hm, sounds like we shouldn't be doing this. I also appear to have a misconception of what SO_REUSEADDR is, so for now I think we should leave these as-is.

I don't like the spurious failures, but with this in mind we should probably pursue another fix.