Enhance signing APIs

The previous signing APIs assumed that a specific key and a digest algorithm were sufficient to determine a signature. With the introduction of RSA-PSS that is no longer the case. A key type of RSA, can produce RSA-PSS-SHA256 or RSA-SHA256 (PKCS#1 v1.5) signatures. We worked around that by introducing specialized flags for the signing functions.

However, to avoid relying on flags, and make the usage of gnutls simpler, we provide a flexible API for signature generation which accepts the actual signature algorithm to be used.