CORE: make sure activated fallback providers stay activated

CORE: make sure activated fallback providers stay activated

Calling 'OSSL_PROVIDER_available(NULL, "default")' would search for
the "default" provider, and in doing so, activate it if necessary,
thereby detecting that it's available...  and then immediately free
it, which could deactivate that provider, even though it should stay
available.

We solve this by incrementing the refcount for activated fallbacks one
extra time, thereby simulating an explicit OSSL_PROVIDER_load(), and
compensate for it with an extra ossl_provider_free() when emptying the
provider store.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11926)