MINOR: backend: move url_param_name/len to lbprm.arg_str/len

This one is exclusively used by LB parameters, when using URL param
hashing. Let's move it to the lbprm struct under a more generic name.
diff --git a/include/types/backend.h b/include/types/backend.h
index b36cd41..ec1f5a9 100644
--- a/include/types/backend.h
+++ b/include/types/backend.h
@@ -144,6 +144,8 @@
 	int tot_used;			/* total number of servers used for LB */
 	int wmult;			/* ratio between user weight and effective weight */
 	int wdiv;			/* ratio between effective weight and user weight */
+	char *arg_str;			/* name of the URL parameter used for hashing */
+	int   arg_len;			/* strlen(arg_str), computed only once */
 	struct server *fbck;		/* first backup server when !PR_O_USE_ALL_BK, or NULL */
 	struct lb_map map;		/* LB parameters for map-based algorithms */
 	struct lb_fwrr fwrr;