encoding/json: add "overflow" struct tag option

Fixes issue 6213.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://codereview.appspot.com/13180043
os/exec: return idempotent Closer from StdinPipe

Before this fix, it was always an error to use the Close method on the
io.WriteCloser obtained from Cmd.StdinPipe, as it would race with the
Close performed by Cmd.Wait.

Fixes issue 6270.

R=golang-dev, r, remyoudompheng, bradfitz, dsymonds
CC=golang-dev
https://codereview.appspot.com/13329043
undo CL 13180043 / 318540e7857f

Accidentally submitted.

??? original CL description
encoding/json: add "overflow" struct tag option

Fixes issue 6213.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://codereview.appspot.com/13180043
???

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/13234045