sysusers: make sysusers work with dnf --installroot

This is not very pretty, but the code in fs-util.c already provisions for
missing /proc. We ourselves are careful to set up /proc, but not everybody
is and it is important for sysusers to also work where shadow-utils would:
I would like to replace calls to useradd and groupadd in Fedora systemd rpm
scriptlets with a call to sysusers. It has a number of advantages:
- dogfooding
- we don't need to manually duplicate the information from our sysusers
  files to scriptlets
- a dependency on shadow-utils is dropped, which transitively drops dependencies
  on setup and fedora-repos and bunch of other stuff.

We could try to get 'dnf' and 'rpm --root' and such to be reworked,
but not in any reasonable timeframe. And even if this was done, we'd still
want to support older rpm/dnf versions.

(cherry picked from commit 82855aa6)