[bug#56994] Acknowledgement ([PATCH] Add nmail)

[Mathieu]

This is my first attempt at packaging for Guix, I hope I didn't do any huge 
mistakes. And thanks IRC for the massive help I've received to slowly progress 
with Scheme.

I was unsure where to add the `use-module` I needed for `nmail.scm` when adding 
it into `mail.scm`, and in the end I think all modules were all already there, 
but to check that, I ordered them alphabetically in `mail.scm` and this 
revealed some duplicates, so the patch reflects that too.