fiwalk: Update rusage reporting

The <runstats> element was an early version of a resource usage report.
Later versions, particularly the one employed by Hashdeep, simply use an
<rusage> element with no renaming of its fields.  This change makes
Fiwalk output a consistent element with Hashdeep and the DFXML C++
library.  (It does not use the DFXML library code included in the same
directory, because that is a less-than-straightforward inclusion. A
future patch should make use of dfxml.{h,cpp} as libraries.)

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>