Added a "fallback" option to be used with the "share" action.

It allows proceeding to another action if a file isn't available.

An example:

    {
        "share": "/data/www/",

        "fallback": {
            "pass": "applications/php"
        }
    }

In the example above, an attempt is made first to serve a request with
a file from the "/data/www/" directory.  If there's no such file, the
request is passed to the "php" application.

Fallback actions may be nested:

    {
        "share": "/data/www/",

        "fallback": {
            "share": "/data/cache/",

            "fallback": {
                "proxy": "http://127.0.0.1:9000"
            }
        }
    }

src/nxt_http.h

@@ -189,6 +189,7 @@ struct nxt_http_action_s {
         nxt_http_route_t            *route;
         nxt_http_upstream_t         *upstream;
         nxt_app_t                   *application;
+        nxt_http_action_t           *fallback;
     } u;
 
     nxt_str_t                       name;