…rite errors

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#42113.

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/+/266158
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

Bring in the change in CL 266158 with:

	go mod edit -replace=golang.org/x/net=golang.org/x/net@release-branch.go1.15-bundle
	GOFLAGS='-mod=mod' go generate -run=bundle std
	go mod edit -dropreplace=golang.org/x/net
	go get -d golang.org/x/net@release-branch.go1.15
	go mod tidy
	go mod vendor

Updates #39337.
Fixes #42113.

Change-Id: I3ebef4b90c11ad271b7a3031aafd80c423c2c241
Reviewed-on: https://go-review.googlesource.com/c/go/+/266375
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>