go.net/ipv6: fix build on dragonfly, plan9, solaris

R=golang-codereviews, aram, r, 0intro
CC=golang-codereviews
https://golang.org/cl/54350043
diff --git a/ipv6/dgramopt_stub.go b/ipv6/dgramopt_stub.go
new file mode 100644
index 0000000..9c760df
--- /dev/null
+++ b/ipv6/dgramopt_stub.go
@@ -0,0 +1,95 @@
+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build dragonfly plan9 solaris
+
+package ipv6
+
+import "net"
+
+// MulticastHopLimit returns the hop limit field value for outgoing
+// multicast packets.
+func (c *dgramOpt) MulticastHopLimit() (int, error) {
+	// TODO(mikio): Implement this
+	return 0, errOpNoSupport
+}
+
+// SetMulticastHopLimit sets the hop limit field value for future
+// outgoing multicast packets.
+func (c *dgramOpt) SetMulticastHopLimit(hoplim int) error {
+	// TODO(mikio): Implement this
+	return errOpNoSupport
+}
+
+// MulticastInterface returns the default interface for multicast
+// packet transmissions.
+func (c *dgramOpt) MulticastInterface() (*net.Interface, error) {
+	// TODO(mikio): Implement this
+	return nil, errOpNoSupport
+}
+
+// SetMulticastInterface sets the default interface for future
+// multicast packet transmissions.
+func (c *dgramOpt) SetMulticastInterface(ifi *net.Interface) error {
+	// TODO(mikio): Implement this
+	return errOpNoSupport
+}
+
+// MulticastLoopback reports whether transmitted multicast packets
+// should be copied and send back to the originator.
+func (c *dgramOpt) MulticastLoopback() (bool, error) {
+	// TODO(mikio): Implement this
+	return false, errOpNoSupport
+}
+
+// SetMulticastLoopback sets whether transmitted multicast packets
+// should be copied and send back to the originator.
+func (c *dgramOpt) SetMulticastLoopback(on bool) error {
+	// TODO(mikio): Implement this
+	return errOpNoSupport
+}
+
+// JoinGroup joins the group address group on the interface ifi.
+// It uses the system assigned multicast interface when ifi is nil,
+// although this is not recommended because the assignment depends on
+// platforms and sometimes it might require routing configuration.
+func (c *dgramOpt) JoinGroup(ifi *net.Interface, group net.Addr) error {
+	// TODO(mikio): Implement this
+	return errOpNoSupport
+}
+
+// LeaveGroup leaves the group address group on the interface ifi.
+func (c *dgramOpt) LeaveGroup(ifi *net.Interface, group net.Addr) error {
+	// TODO(mikio): Implement this
+	return errOpNoSupport
+}
+
+// Checksum reports whether the kernel will compute, store or verify a
+// checksum for both incoming and outgoing packets.  If on is true, it
+// returns an offset in bytes into the data of where the checksum
+// field is located.
+func (c *dgramOpt) Checksum() (on bool, offset int, err error) {
+	// TODO(mikio): Implement this
+	return false, 0, errOpNoSupport
+}
+
+// SetChecksum enables the kernel checksum processing.  If on is ture,
+// the offset should be an offset in bytes into the data of where the
+// checksum field is located.
+func (c *dgramOpt) SetChecksum(on bool, offset int) error {
+	// TODO(mikio): Implement this
+	return errOpNoSupport
+}
+
+// ICMPFilter returns an ICMP filter.
+func (c *dgramOpt) ICMPFilter() (*ICMPFilter, error) {
+	// TODO(mikio): Implement this
+	return nil, errOpNoSupport
+}
+
+// SetICMPFilter deploys the ICMP filter.
+func (c *dgramOpt) SetICMPFilter(f *ICMPFilter) error {
+	// TODO(mikio): Implement this
+	return errOpNoSupport
+}