…rors

When a new connection is created, and a write error occurs during the
initial exchange, the connection must be closed. There is no guarantee
that the caller will close the connection.

When a connection with an existing write error is used or being used, it
will stay in use until its read loop completes. Requests will continue
to use this connection and fail when writing its header. These
connections should be closed to force the cleanup in its readLoop.

Updates golang/go#39337.
For golang/go#42112.

Change-Id: I45e1293309e40629531f4cbb69387864f4f71bc2
Reviewed-on: https://go-review.googlesource.com/c/net/+/240337
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Damien Neil <dneil@google.com>
(cherry picked from commit f585440)
Reviewed-on: https://go-review.googlesource.com/c/net/+/266157
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Fraenkel <michael.fraenkel@gmail.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

Bring in the change in CL 266157 with:

	go get -d golang.org/x/net@release-branch.go1.14
	go mod tidy
	go mod vendor
	go generate -run=bundle std

Updates #39337.
Fixes #42112.

Change-Id: Iefd0012369c7f0c58201256e29d21210cb9f2f7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/266374
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>