enum: Return boolean result for enum_from_name() lookup

Handling the result for enum_from_name() is difficult, as checking for
negative return values requires a cast if the enum type is unsigned. The new
signature clearly differentiates lookup result from lookup value.

Further, this actually allows to convert real -1 enum values, which could not
be distinguished from "not-found" and the -1 return value.

This also fixes several clang warnings where enums are unsigned.

src/libcharon/plugins/vici/vici_query.c

@@ -729,8 +729,7 @@ CALLBACK(list_certs, vici_message_t*,
 	char *str;
 
 	str = request->get_str(request, "ANY", "type");
-	type = enum_from_name(certificate_type_names, str);
-	if (type == -1)
+	if (!enum_from_name(certificate_type_names, str, &type))
 	{
 		b = vici_builder_create();
 		return b->finalize(b);