Package ipv4 implements IP-level socket options for the Internet
Protocol version 4. It also provides raw IP socket access methods
including IPv4 header manipulation.

Fixes golang/go#3684.
Fixes golang/go#3820.

This CL requires CL 6426047;
net: add read, write message methods to IPConn, UDPConn

R=rsc, dave, alex.brainman
CC=gobot, golang-dev
https://golang.org/cl/6482044