lib: Add a helper for mmap->bytes with openat(), use it in repo

lib: Add a helper for mmap->bytes with openat(), use it in repo

This kills another GSystem consumer...I think down the line I'd like
to do something like "detect whether file is > 1k if so, mmap,
otherwise just readall()" so we can use this helper in more places.

Closes: #319
Approved by: jlebon