Modify the tcp_server example to support target_os = "wasi"

Use the `LISTEN_FDS` mechanism to use pre-opened sockets.

Especially for `wasm32-wasi` there is no other way to get access to sockets,
than to use pre-opened sockets.

Because `wasm32-wasi` does not yet return `TcpListener::local_addr()`, an
unspecified IP address and port will be returned and displayed.

```
$ cargo +nightly build --target wasm32-wasi --release --example tcp_server --features="os-poll net"
   Compiling cfg-if v1.0.0
   Compiling wasi v0.10.2+wasi-snapshot-preview1
   Compiling log v0.4.14
   Compiling libc v0.2.112
   Compiling ppv-lite86 v0.2.15
   Compiling wasi v0.11.0+wasi-snapshot-preview1
   Compiling getrandom v0.2.3
   Compiling rand_core v0.6.3
   Compiling env_logger v0.8.4
   Compiling rand_chacha v0.3.1
   Compiling mio v0.8.0 (/home/harald/git/mio)
   Compiling rand v0.8.4
    Finished release [optimized] target(s) in 2.92s

$ wasmtime run --tcplisten 127.0.0.1:9000 --env 'LISTEN_FDS=1' target/wasm32-wasi/release/examples/tcp_server.wasm
Using preopened socket FD 3
You can connect to the server using `nc`:
 $ nc <IP> <PORT>
You'll see our welcome message and anything you type will be printed here.
```

Signed-off-by: Harald Hoyer <harald@profian.com>

examples/tcp_server.rs

@@ -13,6 +13,24 @@ const SERVER: Token = Token(0);
 // Some data we'll send over the connection.
 const DATA: &[u8] = b"Hello world!\n";
 
+#[cfg(not(windows))]
+fn get_first_listen_fd_listener() -> Option<std::net::TcpListener> {
+    #[cfg(unix)]
+    use std::os::unix::io::FromRawFd;
+    #[cfg(target_os = "wasi")]
+    use std::os::wasi::io::FromRawFd;
+
+    let stdlistener = unsafe { std::net::TcpListener::from_raw_fd(3) };
+    stdlistener.set_nonblocking(true).unwrap();
+    Some(stdlistener)
+}
+
+#[cfg(windows)]
+fn get_first_listen_fd_listener() -> Option<std::net::TcpListener> {
+    // Windows does not support `LISTEN_FDS`
+    None
+}
+
 fn main() -> io::Result<()> {
     env_logger::init();
 
@@ -22,8 +40,27 @@ fn main() -> io::Result<()> {
     let mut events = Events::with_capacity(128);
 
     // Setup the TCP server socket.
-    let addr = "127.0.0.1:9000".parse().unwrap();
-    let mut server = TcpListener::bind(addr)?;
+    let mut server = {
+        if std::env::var("LISTEN_FDS").is_ok() && cfg!(not(windows)) {
+            let stdlistener = get_first_listen_fd_listener().unwrap();
+            stdlistener.set_nonblocking(true)?;
+            println!("Using preopened socket FD 3");
+            println!("You can connect to the server using `nc`:");
+            match stdlistener.local_addr() {
+                Ok(a) => println!(" $ nc {} {}", a.ip(), a.port()),
+                Err(_) => println!(" $ nc <IP> <PORT>"),
+            }
+            println!("You'll see our welcome message and anything you type will be printed here.");
+            TcpListener::from_std(stdlistener)
+        } else {
+            let addr = "127.0.0.1:9000".parse().unwrap();
+            let listener = TcpListener::bind(addr)?;
+            println!("You can connect to the server using `nc`:");
+            println!(" $ nc 127.0.0.1 9000");
+            println!("You'll see our welcome message and anything you type will be printed here.");
+            listener
+        }
+    };
 
     // Register the server with poll we can receive events for it.
     poll.registry()
@@ -34,10 +71,6 @@ fn main() -> io::Result<()> {
     // Unique token for each incoming connection.
     let mut unique_token = Token(SERVER.0 + 1);
 
-    println!("You can connect to the server using `nc`:");
-    println!(" $ nc 127.0.0.1 9000");
-    println!("You'll see our welcome message and anything you type will be printed here.");
-
     loop {
         poll.poll(&mut events, None)?;
 
@@ -65,11 +98,8 @@ fn main() -> io::Result<()> {
                     println!("Accepted connection from: {}", address);
 
                     let token = next(&mut unique_token);
-                    poll.registry().register(
-                        &mut connection,
-                        token,
-                        Interest::READABLE.add(Interest::WRITABLE),
-                    )?;
+                    poll.registry()
+                        .register(&mut connection, token, Interest::WRITABLE)?;
 
                     connections.insert(token, connection);
                 },