Remove use of obsolete flags
Bring back the isSystemUser option
Allocate system uids/gids between 400 and 500

Previously it was between 100 and 500, but this can already collide
with the static uids/guid in misc/ids.nix.
Fix some uid/gid attributes to match the actual user/group name
nixos: Add support for changing supported systems

release.nix and release-combined.nix current hardcode the systems which
they are built for. This change introduces an argument to the
expressions called supportedSystems, which allows the builder to choose
which architectures he wants to build. By default, this uses the same
linux x86_64 and i686 architectures.