Break out multicast into a separate package

src/multicast/multicast_unix.go

@@ -0,0 +1,26 @@
+// +build linux netbsd freebsd openbsd dragonflybsd
+
+package multicast
+
+import "syscall"
+import "golang.org/x/sys/unix"
+
+func (m *Multicast) multicastStarted() {
+
+}
+
+func (m *Multicast) multicastReuse(network string, address string, c syscall.RawConn) error {
+	var control error
+	var reuseport error
+
+	control = c.Control(func(fd uintptr) {
+		reuseport = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+	})
+
+	switch {
+	case reuseport != nil:
+		return reuseport
+	default:
+		return control
+	}
+}