[http] Change name of struct field for StreamUrl

Should not be called artwork_url, since it also can link to other ressources.

Also some fixup, e.g. use new macros.

src/http.h

@@ -49,7 +49,7 @@ struct http_icy_metadata
   /* Track specific, comes from icy_metadata_packet */
   char *title;
   char *artist;
-  char *artwork_url;
+  char *url;
 
   uint32_t hash;
 };