crypto/rsa: support > 3 primes.
With full multi-prime support we can support version 1 PKCS#1 private
keys. This means exporting all the members of rsa.PrivateKey, thus
making the API a little messy. However there has already been another
request to export this so it seems to be something that's needed.
Over time, rsa.GenerateMultiPrimeKey will replace rsa.GenerateKey, but
I need to work on the prime balance first because we're no longer
generating primes which are a multiples of 8 bits.
Fixes #987.
R=rsc
CC=golang-dev
https://golang.org/cl/
4378046