go.net/ipv4: restructure ancillary data socket option handling

This CL chops existing ancillary data socket option handlers and
puts them into platform dependent ancillary data socket option
binding table for code readability.

Fixes golang/go#6710.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/153860043
12 files changed