Big revamp in the implmentation of labels/gotos

Added restriction that, when a label is created, there cannot be
another label with the same name visible. That allows backward goto's
to be resolved when they are read. Backward goto's get a close if
they jump out of the scope of some variable; labels get a close only
if previous goto to it jumps out of the scope of some upvalue.

ltests.c

@@ -550,7 +550,7 @@ static char *buildop (Proto *p, int pc, char *buff) {
       sprintf(buff, "%-12s%4d", name, GETARG_Ax(i));
       break;
     case isJ:
-      sprintf(buff, "%-12s%4d (%1d)", name, GETARG_sJ(i), !!GETARG_m(i));
+      sprintf(buff, "%-12s%4d", name, GETARG_sJ(i));
       break;
   }
   return obuff;