certificate: Return signature scheme and parameters from issued_by() method

This also required some include restructuring (avoid including library.h
in headers) to avoid unresolvable circular dependencies.

src/libstrongswan/credentials/credential_manager.h

@@ -241,12 +241,13 @@ struct credential_manager_t {
 	 *
 	 * @param subject	subject certificate to check
 	 * @param issuer	issuer certificate that potentially has signed subject
-	 * @param scheme	receives used signature scheme, if given
+	 * @param scheme	receives used signature scheme and parameters, if
+	 *					given (allocated)
 	 * @return			TRUE if issuer signed subject
 	 */
 	bool (*issued_by)(credential_manager_t *this,
 					  certificate_t *subject, certificate_t *issuer,
-					  signature_scheme_t *scheme);
+					  signature_params_t **scheme);
 
 	/**
 	 * Register a credential set to the manager.