Upgrade from ftw() to nftw()

[NicMcPhee]

To quote man ftw:

ftw() is an older function that offers a subset of the functionality of nftw().

and apparently it's been marked as obsolete.

So we probably replace the use of ftw with nftw here. It looks like it should be pretty simple, as ftw looks to be a simple subset of nftw in most ways.

Thanks to @helloworld12321 for the pointer!

[NicMcPhee]

Actually, now you're supposed to use fts instead of either of the (n)ftw variants.

I got this to work pretty easily (this example was super helpful).

I wonder if this would be a good place to switch to Rust, though, probably using the walkdir crate?

[helloworld12321]

Might be worth mentioning that nftw is POSIX, whereas fts is an extension provided by Linux and a few other Unixes (like macOS). I don't know if that sways things in one direction or another. 🙂🤷🏻‍♂️

(Although a few sources mention that fts is being considered for inclusion in a future POSIX version.)