crypto/x509: allow MS/NS SCG key usage as ServerAuth.

By default, crypto/x509 assumes that users wish to validate
certificates for ServerAuth. However, due to historical reasons,
COMODO's intermediates don't specify ServerAuth as an allowed key
usage.

Rather NSS and CryptoAPI both allow these SGC OIDs to be equivalent to
ServerAuth.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7312070