tools/har-to-old: Replace the "prefix" argument with an optional "destdir"

The previous syntax for har-to-old was:

    usage: har-to-old.py [-h] file prefix

Change it to:

    usage: har-to-old.py [-h] file [destdir]

- prefix renamed in destdir, as it was not really a prefix but the
name(/path) to the destination folder for extracted files
- automatically create the destdir folder if it does not exist
- if destdir is not provided, use the file name without extension