Tidy-up of pruner.pl

- Can be invoked as an executable under unix.
- Avoids dangerous package filehandle and dir handles
- Simplified location of GameData
- Caught error where user may not specify -prune or -unprune
- Simplified file traversal¹
- Code passes `perlcritic --stern`.

¹ I'd recommend using `File::Find` over what we're currently doing; it's
likely faster and easier to understand.