crypto/tls: don't assume an RSA private key in the API.

crypto/tls: don't assume an RSA private key in the API.

We still very much assume it in the code, but with this change in
place we can implement other things later without changing and users
of the package.

Fixes #2319.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5489073