Move more internal URL stuff to RAII.

src/url.hh

@@ -18,6 +18,7 @@
 extern "C++"
 {
 #include <string>
+#include <optional>
 }
 
 
@@ -93,7 +94,7 @@ extern "C" {
 #endif /* __cplusplus */
 
 struct DilloUrl {
-   Dstr  *url_string;
+   std::optional< std::string > url_string;
    const char *buffer;
    const char *scheme;            /**/
    const char *authority;         /**/