Add unshare support
Add unshare (root less) support to the devtools. Most of the work can probably be ported over from the arch-install-scripts at https://gitlab.archlinux.org/archlinux/arch-install-scripts/-/blob/master/common?ref_type=heads.
Mainly makerepropkg, makechrootpkg and arch-nspawn would greatly benefit from this. It looks like implementing unshare support within check_root will be enough, as it is currently the reason most of the tools are currently blocking on sudo asking for the password. arch-nspawn already has (except for the check_root issue) unshare support. The main task is probably to rewrite/update check_root to support unshare.
Is there a reason to not use unshare all the time? If no, we could also avoid to add an additional "-N" parameter for using it and just always use it.