[release-branch.go1.10] crypto/x509: check EKUs like 1.9.

This change brings back the EKU checking from 1.9. In 1.10, we checked
EKU nesting independent of the requested EKUs so that, after verifying a
certifciate, one could inspect the EKUs in the leaf and trust them.

That, however, was too optimistic. I had misunderstood that the PKI was
/currently/ clean enough to require that, rather than it being
desirable. Go generally does not push the envelope on these sorts of
things and lets the browsers clear the path first.

Fixes #25258

Change-Id: I18c070478e3bbb6468800ae461c207af9e954949
Reviewed-on: https://go-review.googlesource.com/113475


Reviewed-by: Filippo Valsorda <filippo@golang.org>
(cherry picked from commit 180e0f8a1b149bd1d15df29b6527748266cacad9)
Reviewed-on: https://go-review.googlesource.com/114035


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>