Previously we would require safe primes for our RSA key generation.
Since this took rather a long time, this removes the requirement that
the primes be safe.

OpenSSL doesn't use safe primes for RSA key generation either
(openssl-0.9.8l/crypto/rsa/rsa_gen.c:122)

Fixes #649.

R=rsc
CC=golang-dev
https://golang.org/cl/253041