Rework some things into packages (#489)

udp/udp_windows.go

@@ -0,0 +1,29 @@
+//go:build !e2e_testing
+// +build !e2e_testing
+
+package udp
+
+// Windows support is primarily implemented in udp_generic, besides NewListenConfig
+
+import (
+	"fmt"
+	"net"
+	"syscall"
+)
+
+func NewListenConfig(multi bool) net.ListenConfig {
+	return net.ListenConfig{
+		Control: func(network, address string, c syscall.RawConn) error {
+			if multi {
+				// There is no way to support multiple listeners safely on Windows:
+				// https://docs.microsoft.com/en-us/windows/desktop/winsock/using-so-reuseaddr-and-so-exclusiveaddruse
+				return fmt.Errorf("multiple udp listeners not supported on windows")
+			}
+			return nil
+		},
+	}
+}
+
+func (u *Conn) Rebind() error {
+	return nil
+}