all: adjust build tags for plan9

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

conn/bind_std.go

@@ -136,6 +136,11 @@ func listenNet(network string, port int) (*net.UDPConn, int, error) {
 	return conn.(*net.UDPConn), uaddr.Port, nil
 }
 
+// errEADDRINUSE is syscall.EADDRINUSE, boxed into an interface once
+// in erraddrinuse.go on almost all platforms. For other platforms,
+// it's at least non-nil.
+var errEADDRINUSE error = errors.New("")
+
 func (s *StdNetBind) Open(uport uint16) ([]ReceiveFunc, uint16, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
@@ -162,7 +167,7 @@ again:
 
 	// Listen on the same port as we're using for ipv4.
 	v6conn, port, err = listenNet("udp6", port)
-	if uport == 0 && errors.Is(err, syscall.EADDRINUSE) && tries < 100 {
+	if uport == 0 && errors.Is(err, errEADDRINUSE) && tries < 100 {
 		v4conn.Close()
 		tries++
 		goto again

conn/controlfns_unix.go

@@ -1,4 +1,4 @@
-//go:build !windows && !linux && !wasm
+//go:build !windows && !linux && !wasm && !plan9
 
 /* SPDX-License-Identifier: MIT
  *

conn/erraddrinuse.go

@@ -0,0 +1,14 @@
+//go:build !plan9
+
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2023 WireGuard LLC. All Rights Reserved.
+ */
+
+package conn
+
+import "syscall"
+
+func init() {
+	errEADDRINUSE = syscall.EADDRINUSE
+}