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.