Fixed to support negative step values for MTFor

It gives support negative step values for MTFor.

**Input:**
```html
Countdown <mt:for from="3" to="1" var="n" glue=" "><mt:var name="n" /></mt:for> Go!!
```
**Output:**

```html
Countdown 3 2 1 Go!!
```

see also `{MT_HOME}/t/35-tags.dat`.

Sorry, I am not update document because my english not good.