Rar multiple volume parsing still gets tricked by filenames matching
new naming prefixes, and using the old naming.

We need to decide according to the number of volumes found if our
new naming scheme has found only 1 volume.

part01.rar,part02.rar
part1.rar,part2.rar
part1.rar,part1.r00 (tricked here)
foobar.rar,foobar.r00

refs #9835