there must be a way to fix versionSuffix for non-flake environments

also: split template applying from the main check
of existence of configuration.nix

… manpage

Otherwise we'd run into trouble when a filename contained e.g. a space

To more accurately represent what it does

Opens the file that would be built from

Previously, `nixos-rebuild -A foo` would build the `default.nix` in the
current directory, while `nixos-rebuild` without `-A` would not use
file-based building at all.

This difference compared to what `nix-build` would do is confusing.

Furthermore, using `default.nix` is not great, because there's many of
those files all around, and they're almost never for NixOS.

This commit fixes those two issues by:
- Removing support for using `default.nix` as a default when using `--attr`
- If it exists, using the `<nixos-system>` entry in NIX_PATH as the default
- Otherwise, use the `system.nix` file in the current directory or any
  of its parents, if it exists, as a default
- Otherwise, use `/etc/nixos/system.nix` as a default

While `system.nix` could be conflated with the `system` Nixpkgs
argument, there's no practical way for this to cause problems, because
`system` is never specified in a file.

Having this finally allows using `nixos-rebuild` without any arguments
and without any `NIX_PATH` entries, effectively a way to declaratively
use NixOS from just /etc/nixos!