Previously we enforced both that the extended key usages of a client
certificate chain allowed for client authentication, and that the
client-auth EKU was in the leaf certificate.

This change removes the latter requirement. It's still the case that the
chain must be compatible with the client-auth EKU (i.e. that a parent
certificate isn't limited to another usage, like S/MIME), but we'll now
accept a leaf certificate with no EKUs for client-auth.

While it would be nice if all client certificates were explicit in their
intended purpose, I no longer feel that this battle is worthwhile.

Fixes #11087.

Change-Id: I777e695101cbeba069b730163533e2977f4dc1fc
Reviewed-on: https://go-review.googlesource.com/10806
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>