Change to run mtools under debugger in #427 introduces regression in stdin support

Prior to #427, piping a log from stdin works, eg:

```
mlogfilter mongod.log --slow | mplotqueries
```

With #427 applied this now returns:

```
no data to plot.
```

Since this is a usage regression from prior releases, we should back out the change and work out how to add compatible debugging support.
