<enter reason for undo>

««« original CL description
net: make use of SO_LINGER_SEC on darwin

Fixes #7971.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/92210044
»»»

TBR=iant
R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/96220049