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/eap_radius/eap_radius_forward.c

@@ -362,8 +362,7 @@ static linked_list_t* parse_selector(char *selector)
 			vendor = atoi(token);
 			token = pos;
 		}
-		type = enum_from_name(radius_attribute_type_names, token);
-		if (type == -1)
+		if (!enum_from_name(radius_attribute_type_names, token, &type))
 		{
 			type = atoi(token);
 		}