Initialize the xkey provider and use it in SSL context

Initialize the xkey provider and use it in SSL context

- Add function to check when external key is in use

- Load xkey provider into a custom library context when required

- Use the custom libctx in SSL CTX when external key is in use

As no keys are yet loaded through the provider,
no functionality gets delegated to it as yet.

v2 changes: Provider loading is reworked to activate only when external
            keys are in use
            This was 2/9 in v1

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-6-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23432.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>