Refactor strong os-RNG into its own function
Previously, we only used the strong OS entropy source as part of seeding OpenSSL's RNG. But with curve25519, we'll have occasion to want to generate some keys using extremely-good entopy, as well as the means to do so. So let's! This patch refactors the OS-entropy wrapper into its own crypto_strongest_rand() function, and makes our new curve25519_secret_key_generate function try it as appropriate.