Nvml mjgrabows log benchmark

This is a multi-threaded benchmark for the pmemlog mode.
Function header comments should be:

```
/*
 * function_name -- short description
 *
 * Optional English sentences with more info.
 */
```

In other words, the short description must be short enough that the it fits on the line and doesn't require continuation on the next line.  If you can't fit it, you're not describing it briefly enough.  Additional details are then optionally added as complete English sentences after the blank comment line.  (Fix this and the others as well.)