For:

    http2: track reused connections
    https://golang.org/cl/176720 (updates golang/go#31982)

Some x/sys/unix updates come along for the ride too.

I filed #32031 for making the bundling process less difficult and
error-prone in the future.

Change-Id: Ic822080991ffa2d50352c5f613e45648a327cf16
Reviewed-on: https://go-review.googlesource.com/c/go/+/177037
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>