deps: allow crypto/x509 cgo and io/ioutil.
deps: allow crypto/x509 cgo and io/ioutil.

In order to land 5700087 (which moves the knowledge of how to get the
root certificates for the system from crypto/tls to crypto/x509), we
need to relax the restrictions on crypto/x509. Afterwards, we can
probably tighten them up in crypto/tls.

R=golang-dev, rsc, krautz
CC=golang-dev
http://codereview.appspot.com/5753060

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