net: separate NaCl dependent placeholders from BSD's

To clarify the dependency of NaCl platform.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/143830044
diff --git a/src/net/sockoptip_stub.go b/src/net/sockoptip_stub.go
index dcd3a22..32ec5dd 100644
--- a/src/net/sockoptip_stub.go
+++ b/src/net/sockoptip_stub.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build solaris
+// +build nacl solaris
 
 package net
 
@@ -10,30 +10,30 @@
 
 func setIPv4MulticastInterface(fd *netFD, ifi *Interface) error {
 	// See golang.org/issue/7399.
-	return syscall.EINVAL
+	return syscall.ENOPROTOOPT
 }
 
 func setIPv4MulticastLoopback(fd *netFD, v bool) error {
 	// See golang.org/issue/7399.
-	return syscall.EINVAL
+	return syscall.ENOPROTOOPT
 }
 
 func joinIPv4Group(fd *netFD, ifi *Interface, ip IP) error {
 	// See golang.org/issue/7399.
-	return syscall.EINVAL
+	return syscall.ENOPROTOOPT
 }
 
 func setIPv6MulticastInterface(fd *netFD, ifi *Interface) error {
 	// See golang.org/issue/7399.
-	return syscall.EINVAL
+	return syscall.ENOPROTOOPT
 }
 
 func setIPv6MulticastLoopback(fd *netFD, v bool) error {
 	// See golang.org/issue/7399.
-	return syscall.EINVAL
+	return syscall.ENOPROTOOPT
 }
 
 func joinIPv6Group(fd *netFD, ifi *Interface, ip IP) error {
 	// See golang.org/issue/7399.
-	return syscall.EINVAL
+	return syscall.ENOPROTOOPT
 }