net/http: use TCP keep-alives for ListenAndServe and ListenAndServeTLS
net/http: use TCP keep-alives for ListenAndServe and ListenAndServeTLS

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://codereview.appspot.com/48300043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang