PHP: implemented "targets" option.

This allows to specify multiple subsequent targets inside PHP applications.
For example:

  {
      "listeners": {
          "*:80": {
              "pass": "routes"
          }
      },

      "routes": [
          {
              "match": {
                  "uri": "/info"
              },

              "action": {
                  "pass": "applications/my_app/phpinfo"
              }
          },
          {
              "match": {
                  "uri": "/hello"
              },

              "action": {
                  "pass": "applications/my_app/hello"
              }
          },
          {
              "action": {
                  "pass": "applications/my_app/rest"
              }
          }
      ],

      "applications": {
          "my_app": {
              "type": "php",
              "targets": {
                  "phpinfo": {
                      "script": "phpinfo.php",
                      "root": "/www/data/admin",
                  },

                  "hello": {
                      "script": "hello.php",
                      "root": "/www/data/test",
                  },

                  "rest": {
                      "root": "/www/data/example.com",
                      "index": "index.php"
                  },
              }
          }
      }
  }

src/nxt_router.h

@@ -133,8 +133,11 @@ struct nxt_app_s {
     nxt_nsec_t             res_timeout;
     nxt_msec_t             idle_timeout;
 
+    nxt_str_t              *targets;
+
     nxt_app_type_t         type:8;
 
+    nxt_mp_t               *mem_pool;
     nxt_queue_link_t       link;
 
     nxt_str_t              conf;