Add functions to see if a provider is available for use.

Add functions to see if a provider is available for use.

Public function OSSL_PROVIDER_available() takes a library context and
a provider name, and returns 1 if it's available for use, i.e. if it's
possible to fetch implementations from it, otherwise 0.

Internal function ossl_provider_activated() returns 1 if the given
OSSL_PROVIDER is activated, otherwise 0.

To make this possible, the activation of fallbacks got refactored out
to a separate function, which ended up simplifying the code.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9398)