* Verify CA from on-disk file
Revert part of #213 again, because `SSL_CTX_load_verify_locations`
is just more reliable at setting up the trust store.
It looks like it's able to reference the .pem multiple times in
those cases where the root issuer of the CA is also embedded in
the file (which is the case with e.g. Let's Encrypt).
This is better than the current implementation, and I couldn't
easily find a way to do the same in-memory. I'd rather use the
standard API here.
See 7a85d3cac7
Minimize target dependency on OpenSSL (easier to drop later).
Outside of OpenVPN tunnel extension, OpenSSL is only used to
decrypt encrypted private keys in CryptoContainer (found in
TunnelKitOpenVPNCore, therefore "temporarily" dependent on
CTunnelKitOpenVPNAppExtension for TLSBox/CryptoBox).
Davide De Rosa