BUG/MAJOR: fwlc: Count an avoided server as unusable.

When fwlc_get_next_server(), if a server to avoid has been provided, and
we have to ignore it, don't forget to increase the number of unusable
servers, otherwise we may end up ignoring it over and over, never
switching to another server, in an infinite loop until the process gets
killed.
This hopefully fixes Github issues #3004 and #3014.

This should be backported to 3.2.

src/lb_fwlc.c

@@ -806,6 +806,7 @@ struct server *fwlc_get_next_server(struct proxy *p, struct server *srvtoavoid)
                                                 break;
                                         }
 					avoided = s;
+					unusable++;
 				}
 				else
 					unusable++;