Fix use of file finders

- extend the scanning of runtime prerequisites to include executables
- document the file finder options
- renamed the file finder options to indicate what they mean, not their present defaults
- reverted the reversion of the filename check - with this in place, it is IMPOSSIBLE to scan executables that don't have a .pm/.pl suffix - e.g. https://metacpan.org/pod/distribution/App-Uni/bin/uni