openssl: add engine method for loading the key

openssl: add engine method for loading the key

As well as doing crypto acceleration, engines can also be used to load
key files.  If the engine is set, and the private key loading fails
for bio methods, this patch makes openvpn try to get the engine to
load the key.  If that succeeds, we end up using an engine based key.
This can be used with the openssl tpm engines to make openvpn use a
TPM wrapped key file.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200528225920.6983-2-James.Bottomley@HansenPartnership.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19937.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>