Allow loading of non default providers

Allow loading of non default providers

This allows OpenVPN to load non-default providers. This is mainly
useful for loading the legacy provider with --providers legacy default

Patch v4: use spaces to seperate providers, unload providers.
Patch v5: General cleanup, rename option to --providers, add
          option to usage() and add an entry to Changes.rst
Patch v6: allow --providers also to be used (and be ignored) with mbed TLS

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211112130231.3799480-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/search?l=mid&q=20211112130231.3799480-1-arne@rfc2549.org
Signed-off-by: Gert Doering <gert@greenie.muc.de>