MINOR: config: emit a warning when 'default_backend' masks servers

When a "listen" instance uses a "default_backned" rule and has servers,
the servers will never be used. Report it so that users don't get trapped.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 815ffff..77f0c5d 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -5723,6 +5723,13 @@
 				 */
 				target->bind_proc = curproxy->bind_proc ?
 					(target->bind_proc | curproxy->bind_proc) : 0;
+
+				/* Emit a warning if this proxy also has some servers */
+				if (curproxy->srv) {
+					Warning("In proxy '%s', the 'default_backend' rule always has precedence over the servers, which will never be used.\n",
+						curproxy->id);
+					err_code |= ERR_WARN;
+				}
 			}
 		}