go.net/ipv6: add deadline methods to PacketConn
go.net/ipv6: add deadline methods to PacketConn

Update issue 5538

R=dave, r
CC=golang-dev
https://codereview.appspot.com/10089046

go-mirrors/go.net,polydawn-ports/go.net,gigq/go.net,ssor/go.net,fernandez14/go.net