Unfortunately, gnutls_pubkey_verify_data() is deprecated. Which is a pain;
the 'threat model' that led to that deprecation doesn't apply here, and it
just means we have to jump through hoops to find the 'intended' algorithm
instead of letting it be inferred from the signature.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

Slightly reduce the #ifdef hell in gnutls.c
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>