Replumbing: add fallback provider capability

To ensure that old applications aren't left without any provider, and
at the same time not forcing any default provider on applications that
know how to deal with them, we device the concept of fallback
providers, which are automatically activated if no other provider is
already activated.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8480)