Field 'Proto.is_vararg' uses only one bit

So that the other bits can be used for other purposes.

lparser.c

@@ -959,7 +959,7 @@ static void constructor (LexState *ls, expdesc *t) {
 
 
 static void setvararg (FuncState *fs, int nparams) {
-  fs->f->is_vararg = 1;
+  fs->f->flag |= PF_ISVARARG;
   luaK_codeABC(fs, OP_VARARGPREP, nparams, 0, 0);
 }
 
@@ -1177,7 +1177,7 @@ static void simpleexp (LexState *ls, expdesc *v) {
     }
     case TK_DOTS: {  /* vararg */
       FuncState *fs = ls->fs;
-      check_condition(ls, fs->f->is_vararg,
+      check_condition(ls, fs->f->flag & PF_ISVARARG,
                       "cannot use '...' outside a vararg function");
       init_exp(v, VVARARG, luaK_codeABC(fs, OP_VARARG, 0, 0, 1));
       break;