[release-branch.go1.8] crypto/x509: reject intermediates with unknown critical extensions.

In https://golang.org/cl/9390 I messed up and put the critical extension
test in the wrong function. Thus it only triggered for leaf certificates
and not for intermediates or roots.

In practice, this is not expected to have a security impact in the web
PKI.

[Merge conflicts resolved in verify_test.go]

Change-Id: I4f2464ef2fb71b5865389901f293062ba1327702
Reviewed-on: https://go-review.googlesource.com/69294


Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/70842


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>