Initialization design about libsodium

[jedisct1]

A normal lock means introducing a dependency on pthreads, and I really want to avoid that.

reinit() and shutdown() are gone.

init() can be called more than once but is not thread-safe. It probably shouldn't even try to be.

Actually all these issues with init are due to the fact that the current API doesn't take any context.
So another way to go is to replace all the crypto__() functions with sodium__(, ...) and provide crypto_*() wrappers that always use the reference implementations.