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;
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 14b6046..39e9940 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -320,8 +320,6 @@
 	unsigned int cookie_maxlife;		/* max life time for this cookie */
 	char *rdp_cookie_name;			/* name of the RDP cookie to look for */
 	int  rdp_cookie_len;			/* strlen(rdp_cookie_name), computed only once */
-	char *url_param_name;			/* name of the URL parameter used for hashing */
-	int  url_param_len;			/* strlen(url_param_name), computed only once */
 	int  uri_len_limit;			/* character limit for uri balancing algorithm */
 	int  uri_dirs_depth1;			/* directories+1 (slashes) limit for uri balancing algorithm */
 	int  uri_whole;				/* if != 0, calculates the hash from the whole uri. Still honors the len_limit and dirs_depth1 */