“too many open files” error in awk

I am running `pmonitor -i1` on a long-running `cp` process that copies some large files but also very many small files, and I’m getting this error:

```
awk: cmd. line:4: (FILENAME=- FNR=18787) fatal: cannot open pipe `date +%s' (Too many open files)
```

I think `file_length` is leaking file descriptors (though I’m not quite sure about the details).

PR incoming.