Fixes non-random behaviour when randomizing file lists several times per
second.

Closes #349

Note that the i == r check was never reached in the original code but may be
in the new variant. Since a shuffled list may well have one or two elements
retain their absolute position once in a while, this should not be a problem.

tests with 2- and 3-element lists show an even distribution amongst all
elements.

Not all functions were examined yet. Also TODO:
* check strdup / malloc calls (some are apparently unchecked in giblib),
  maybe use estrdup / emalloc for those
* Check includes and update dependency list if needed