Always load intermediate certificates from a PKCS#12 file

Load intermediate certificates from a PKCS#12 file and place them in the
extra certs chain, when trusted CA certs are loaded from an external PEM
file with the --ca option, and the CA certs in PKCS#12 are not to be trusted.

Required when client PKCS#12 file is provided by a different CA
than the server CA, the PKCS#12 file contains intermediate certificates
required for client auth, but the server CA is not in the PKCS#12 file.

When --ca is set, the PKCS#12 provided CA certs are not trusted. Without
this patch, they were ignored completely - with this patch, they're loaded
in the extra certs chain which makes them available for chain verification
but still does not make them trusted if --ca is set. Unless when, of
course, a trusted root is found from the --ca file.

Acked-by: James Yonan <james@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <alpine.DEB.2.02.1306201400320.10116@jazz.he.fi>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7721

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6481f879eb62cafa6ad652801b2b5c45e546ef44)