Fix: "invalid literal for float()" error when parsing `fdisk` output

Reason was the wrong locale setting. We need to set LC_ALL=C, too.
Change date in infotext into ISO-format (yyyy-mm-dd).

ISO-format saves showing the used format beside the data since only
the ISO-format looks like this.
Use sys.executable for `callself` instead of hard-coded path.

This is for Linux only. I did not dare touching the Windows part,
as it does too much stuff I not understand why it is done.