Preserve old API by using correct struct in system call
and then copying the results, as we did for SetsockoptLinger.
R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6307065
This quiets all.bash noise for upcoming features we know about.
The all.bash warnings will now only print for things not in next.txt
(or in next.txt but not in the API).
Once an API is frozen, we rename next.txt to a new frozen file
(like go1.txt)
Fixes#3651
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6218069
