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 and RawConn 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
and Header can help to marshal and parse information contained in IO
messages.

Updates golang/go#3661.

Change-Id: Ia84a9d3bc51641406eaaf4258f2a3066945cc323
Reviewed-on: https://go-review.googlesource.com/38275
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>