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_unit_request.h

@@ -21,6 +21,7 @@ struct nxt_unit_request_s {
     uint8_t               local_length;
     uint8_t               tls;
     uint8_t               websocket_handshake;
+    uint8_t               app_target;
     uint32_t              server_name_length;
     uint32_t              target_length;
     uint32_t              path_length;