Correct polarity of packet mode bit.

Problem discovered when 'boot0cfg -B -v -o packet -s 2 ad4' turned off
packet mode while reporting it as being enabled.

usr.sbin/boot0cfg/boot0cfg.c

@@ -61,7 +61,7 @@ static const struct {
     const char *tok;
     int def;
 } opttbl[] = {
-    {"packet", 1},
+    {"packet", 0},
     {"update", 1},
     {"setdrv", 0}
 };