[RARSLAVE] Improve "likely file" detection

Improve the likely file detection by using not only the name of the par2
files, but also what they are protecting. This should help a lot.

Update all uses of find_likely_files() to the new API.

Also, fix the filetype detectors so that they work, and move the duplicate
code to a new function.

Signed-off-by: Ira W. Snyder <devel@irasnyder.com>