This change provides message IO functionality that may support the
construction of modern datagram transport protocols.

The modern datagram transport protocols on a multihomed node basically
need to control each packet path for traffic engineering by using
information belongs to network- or link-layer implementation. In
addtion, it's desirable to be able to do simultaneous transmission
across multiple network- or link-layer adjacencies wihtout any
additional cost.

The ReadBatch and WriteBatch methods of PacketConn can be used to read
and write an IO message that contains the information of network- or
link-layer implementation, and read and write a batch of IO messages
on Linux. The Marshal and Parse methods of ControlMessage can help to
marshal and parse information contained in IO messages.

Updates golang/go#3661.

Change-Id: I94484e2e135f4969ddf5c2548bf6be0cc16888bf
Reviewed-on: https://go-review.googlesource.com/38276
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>