rpki-client: simplify signature type checking for certs/CRLs

rpki-client: simplify signature type checking for certs/CRLs

The OpenSSL 1.1 get_signature_nid() API is available for all libraries
that we support and it does exactly what we want. It is much simpler
than the unergonomic accessors we used previously. The ASN.1 templates
ensure that the relevant struct members aren't NULL after successful
deserialization, so the calls are safe.

ok claudio