Document SO_REUSEADDR to be the default for TCP `bind`

[badeend]

For TCP sockets specifically, this is a reasonable default for WASI to recommend:

• For the vast majority of cases, this is "what you want"
• On Windows this is the default behavior and can't be opted out of. In other words: every existing cross-platform application already has to deal with this behavior.
• This makes it easier to implement wasi-sockets on top of standard libraries that set SO_REUSEADDR by default too. Examples:
  • https://github.com/rust-lang/rust/blob/e51e98dde6a60637b6a71b8105245b629ac3fe77/library/std/src/sys_common/net.rs#L408
  • https://github.com/libuv/libuv/blob/v1.x/src/unix/tcp.c#L167
  • https://github.com/dotnet/runtime/blob/23e5d5410ed3e8bc9ab61b3685e1d3a06bdba701/src/native/libs/System.Native/pal_networking.c#L1566
  • https://github.com/golang/go/blob/master/src/net/sockopt_linux.go#L28
• This does not preclude us from adding an advanced option in the future to opt-out of this.

---

This is the behavior as implemented by Wasmtime and JCO.

[badeend]

@sunfishcode Do you agree?