APPS: Make it possible for apps to set the base (fallback) UI_METHOD

APPS: Make it possible for apps to set the base (fallback) UI_METHOD

The apps UI method acts as a proxy that bases its activity on a base
(was called fallback) UI_METHOD, which defaults to UI_OpenSSL() under
normal circumstances.

However, some apps might want to have it based on another UI_METHOD,
such as UI_null() to avoid prompting (typical for a -batch run).  The
new function set_base_ui_method() allows them to do precisely this.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/13512)