net: enable UDP broadcast before it is needed (instead of after)

Fixes #526.

R=r
CC=golang-dev
https://golang.org/cl/186211

src/pkg/net/sock.go

@@ -35,6 +35,9 @@ func socket(net string, f, p, t int, la, ra syscall.Sockaddr, toAddr func(syscal
 	// Allow reuse of recently-used addresses.
 	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
 
+	// Allow broadcast.
+	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1)
+
 	if la != nil {
 		e = syscall.Bind(s, la)
 		if e != 0 {

src/pkg/net/udpsock.go

@@ -71,11 +71,7 @@ type UDPConn struct {
 	fd *netFD
 }
 
-func newUDPConn(fd *netFD) *UDPConn {
-	c := &UDPConn{fd}
-	setsockoptInt(fd.sysfd, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1)
-	return c
-}
+func newUDPConn(fd *netFD) *UDPConn { return &UDPConn{fd} }
 
 func (c *UDPConn) ok() bool { return c != nil && c.fd != nil }