crypto/tls: explicitly require ExtKeyUsageClientAuth for client certs
crypto/tls: explicitly require ExtKeyUsageClientAuth for client certs

If we aren't explicit about the KeyUsages, the verifier
will treat the certificate as a server certificate and require
it to have a ExtKeyUsageServerAuth key usage.

R=golang-dev
CC=golang-dev
http://codereview.appspot.com/6453148

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

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