Fix pkcs7 chain building

[oliwel]

The code in the pkcs7_get_chain method of the default token is broken. It tries do build the chain using the issuer subject string. This fails for multivalued CNs as pkcs7 and openssl use different formats on stringified subjects.

CN=Oliver Welter/UID=oliwel,DC=OpenXPKI,DC=org vs.
UID=oliwel+CN=Oliver Welter,DC=OpenXPKI,DC=org

As this also can lead to ambigouty, suggested fix is to use AIA info as in certnanny to construct the chain https://github.com/certnanny/CertNanny/blob/master/agent/lib/perl/CertNanny/Keystore.pm#L1342

A continous formatting needs to be defined and implemented with respect to other issues and as a fallback.