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.