Implemented "location" option for "return" action.

This allows to specify redirects:

  {
      "action": {
          "return": 301,
          "location": "https://www.example.com/"
      }
  }

src/nxt_http.h

@@ -23,6 +23,8 @@ typedef enum {
     NXT_HTTP_FOUND = 302,
     NXT_HTTP_SEE_OTHER = 303,
     NXT_HTTP_NOT_MODIFIED = 304,
+    NXT_HTTP_TEMPORARY_REDIRECT = 307,
+    NXT_HTTP_PERMANENT_REDIRECT = 308,
 
     NXT_HTTP_BAD_REQUEST = 400,
     NXT_HTTP_FORBIDDEN = 403,