SSL: disabled UI console prompts from worker processes.

SSL: disabled UI console prompts from worker processes.

Certain providers may attempt to reload the key on the first use after a
fork.  Such attempt would require re-prompting the pin, and this time we
are not able to pass the password callback.

While it is addressable with configuration for a specific provider, it would
be prudent to ensure that no such prompts could block worker processes by
setting the default UI method.

UI_null() first appeared in 1.1.1 along with the OSSL_STORE, so it is safe
to assume the same set of guards.