Add option filter to return_dir_as_array()

- Add a 2nd parameter, default empty, that allows to pass a regex to
  filter files it'll return
- While I'm here, simplify code removinf $canadd var
Fix rrd databases upgrade. Helps ticket #2651

- Replace find that was being called with wrong parameters at
upgrade_054_to_055() and upgrade_080_to_081() by return_dir_as_array()
call.