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
diff --git a/ipv6/icmp_stub.go b/ipv6/icmp_stub.go
index 58b6a04..c1263ec 100644
--- a/ipv6/icmp_stub.go
+++ b/ipv6/icmp_stub.go
@@ -9,7 +9,10 @@
 type sysICMPv6Filter struct {
 }
 
-func (f *sysICMPv6Filter) set(typ ICMPType, block bool) {
+func (f *sysICMPv6Filter) accept(typ ICMPType) {
+}
+
+func (f *sysICMPv6Filter) block(typ ICMPType) {
 }
 
 func (f *sysICMPv6Filter) setAll(block bool) {