net: support error.Is of network errors and context errors

net: support error.Is of network errors and context errors

Change timeouts to be Is(context.DeadlineExceeded) and cancelation to
be Is(context.Canceled).

Fixes #51428

Change-Id: Ic580bd9da0f338e993fb79138875a78d99cc1a1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/396877
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>