Add Socket.peek to allow peeking at the receiving queue. (#1933)

* Add `Socket.peek` to allow peeking at the receiving queue.

This uses the `MSG_PEEK` flag to peek at the beginning of the
receiving queue without removing data from said queue.

lib/std/net/socket.c3

@@ -156,6 +156,16 @@ fn void! Socket.close(&self) @inline @dynamic
 	self.sock.close()!;
 }
 
+fn usz! Socket.peek(&self, char[] bytes) @dynamic
+{
+  $if env::WIN32:
+	  isz n = libc::recv(self.sock, bytes.ptr, (int)bytes.len, os::MSG_PEEK);
+  $else
+	  isz n = libc::recv(self.sock, bytes.ptr, bytes.len, os::MSG_PEEK);
+  $endif
+	if (n < 0) return os::socket_error()?;
+	return (usz)n;
+}
 
 enum SocketShutdownHow : (inline CInt native_value)
 {