Our default behavior for the common cases shouldn't lead to
leaked TCP connections (e.g. from people closing laptops) when
their Go servers are exposed to the open Internet without a
proxy in front.

Too many users on golang-nuts have learned this the hard way.

No API change. Only ListenAndServe and ListenAndServeTLS are
updated.

R=golang-codereviews, cespare, gobot, rsc, minux.ma
CC=golang-codereviews
https://golang.org/cl/48300043