crypto/x509: tighten EKU checking for requested EKUs.

crypto/x509: tighten EKU checking for requested EKUs.

There are, sadly, many exceptions to EKU checking to reflect mistakes
that CAs have made in practice. However, the requirements for checking
requested EKUs against the leaf should be tighter than for checking leaf
EKUs against a CA.

Fixes #23884

Change-Id: I05ea874c4ada0696d8bb18cac4377c0b398fcb5e
Reviewed-on: https://go-review.googlesource.com/96379
Reviewed-by: Jonathan Rudenberg <jonathan@titanous.com>
Reviewed-by: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Filippo Valsorda <hi@filippo.io>
TryBot-Result: Gobot Gobot <gobot@golang.org>