[release-branch.go1.10] crypto/x509: matching any requested EKU should be sufficient.

The documentation was unclear here and I misremembered the behaviour and
changed it in 1.10: it used to be that matching any EKU was enough but
1.10 requires that all EKUs match.

Restore 1.9 behaviour and clarify the documentation to make it official.

Fixes #24162.

Change-Id: Ic9466cd0799cb27ec3a3a7e6c96f10c2aacc7020
Reviewed-on: https://go-review.googlesource.com/97720


Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-on: https://go-review.googlesource.com/102790
Run-TryBot: Andrew Bonventre <andybons@golang.org>