[BUG] option allbackups was not working anymore in roundrobin mode

Commit 3168223a7b33a1d5aad1e11b8f2ad917645d7f27 broke option
"allbackups" in roundrobin mode due to an erroneous structure
member replacement in backend.c. The PR_O_USE_ALL_BK flag was
not tested in the right member anymore.

This bug uncoverred another one, by which all backup servers would
be used whatever the option's value, if all of them had been seen
as simultaneously failed at one moment.

This patch fixes the two stupid errors. Correctness has been tested
using the test-fwrr.cfg config example.
diff --git a/src/backend.c b/src/backend.c
index 1cd2a19..666c218 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -70,7 +70,7 @@
  * This function recounts the number of usable active and backup servers for
  * proxy <p>. These numbers are returned into the p->srv_act and p->srv_bck.
  * This function also recomputes the total active and backup weights. However,
- * it does nout update tot_weight nor tot_used. Use update_backend_weight() for
+ * it does not update tot_weight nor tot_used. Use update_backend_weight() for
  * this.
  */
 static void recount_servers(struct proxy *px)
@@ -86,7 +86,7 @@
 
 		if (srv->state & SRV_BACKUP) {
 			if (!px->srv_bck &&
-			    !(px->lbprm.algo & PR_O_USE_ALL_BK))
+			    !(px->options & PR_O_USE_ALL_BK))
 				px->lbprm.fbck = srv;
 			px->srv_bck++;
 			px->lbprm.tot_wbck += srv->eweight;
@@ -376,16 +376,21 @@
 		p->lbprm.tot_wbck = p->lbprm.fwrr.bck.next_weight;
 		p->srv_bck++;
 
-		if (p->lbprm.fbck) {
-			/* we may have restored a backup server prior to fbck,
-			 * in which case it should replace it.
-			 */
-			struct server *srv2 = srv;
-			do {
-				srv2 = srv2->next;
-			} while (srv2 && (srv2 != p->lbprm.fbck));
-			if (srv2)
+		if (!(p->options & PR_O_USE_ALL_BK)) {
+			if (!p->lbprm.fbck) {
+				/* there was no backup server anymore */
 				p->lbprm.fbck = srv;
+			} else {
+				/* we may have restored a backup server prior to fbck,
+				 * in which case it should replace it.
+				 */
+				struct server *srv2 = srv;
+				do {
+					srv2 = srv2->next;
+				} while (srv2 && (srv2 != p->lbprm.fbck));
+				if (srv2)
+					p->lbprm.fbck = srv;
+			}
 		}
 	} else {
 		p->lbprm.tot_wact = p->lbprm.fwrr.act.next_weight;