add some bounds, for a subsequent commit:

• all writers of exstat ensure the value is in [0; 0xFF]
• all readers of exstat AND it with 0xFF (not strictly needed thus)
• trap_exstat is “safe”, i.e. always either -1 or [0; 0xFF]

var.c

@@ -27,7 +27,7 @@
 #include <sys/sysctl.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/var.c,v 1.153 2012/07/30 21:37:17 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/var.c,v 1.154 2012/10/21 21:39:06 tg Exp $");
 
 /*-
  * Variables
@@ -260,7 +260,7 @@ global(const char *n)
 				vp->flag &= ~(ISSET|INTEGER);
 			break;
 		case '?':
-			vp->val.i = exstat;
+			vp->val.i = exstat & 0xFF;
 			break;
 		case '#':
 			vp->val.i = l->argc;