crypto/x509: remove explicit uses of rsa.

crypto/x509: remove explicit uses of rsa.

(Sending to r because of the API change.)

Over time we might want to add support for other key types.

While I was in the code, I also made the use of RawSubject the same
between Subject and Issuer when creating certificates.

R=r, rsc
CC=golang-dev
https://golang.org/cl/5554049