Convert the existing X.509 certificate plugin to use QCA2, instead of KSSL.

This plugin can now handle X.509 Certificate, Certificate signing requests and Certificate Revocation Lists (CRLs). Certificates are handled in both PEM and DER, and the other two are handled in PEM format (DER wouldn't be hard to add, but there doesn't appear to be any real-world usage). Signing Requests and CRLs provide less information than a Certificate (e.g. the Signing Request doesn't have any issuer information).

The amount of information that can be provided is now extended to include multiple DNS names and XMPP information (per RFC3290), plus the State/Province (if any) for both issuer and subject.

Adding URI or IPAddress would be trivial if there was any real-world need for it.

The old CMakeLists only compiled this if SASL was found - not sure why. In any case, it now only gets compiled if QCA2 is found.