The previous implementation used per-socket TCP keepalive options
wrong. For example, it used another level socket option to control
TCP and it didn't use TCP_KEEPINTVL option when possible.

Fixes #8683.
Fixes #8701.
Update #8679

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/136480043