x/net/ipv6: split Set method of ICMPFilter into Accept, Block methods

This is an API breaking change.

method (*ICMPFilter) Set(ICMPType, bool)

is now replaced with

method (*ICMPFilter) Accept(ICMPType)
method (*ICMPFilter) Block(ICMPType)

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/182450043
10 files changed