net: disable wild use of SO_REUSEPORT on BSD variants

Fixes #2830 (again).

R=rsc
CC=golang-dev
https://golang.org/cl/5651083

src/pkg/net/sockopt_bsd.go

@@ -34,14 +34,6 @@ func setDefaultListenerSockopts(s int) error {
 	if err != nil {
 		return os.NewSyscallError("setsockopt", err)
 	}
-	// Allow reuse of recently-used ports.
-	// This option is supported only in descendants of 4.4BSD,
-	// to make an effective multicast application and an application
-	// that requires quick draw possible.
-	err = syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEPORT, 1)
-	if err != nil {
-		return os.NewSyscallError("setsockopt", err)
-	}
 	return nil
 }
 
@@ -52,6 +44,10 @@ func setDefaultMulticastSockopts(s int) error {
 	if err != nil {
 		return os.NewSyscallError("setsockopt", err)
 	}
+	// Allow reuse of recently-used ports.
+	// This option is supported only in descendants of 4.4BSD,
+	// to make an effective multicast application that requires
+	// quick draw possible.
 	err = syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEPORT, 1)
 	if err != nil {
 		return os.NewSyscallError("setsockopt", err)