installer: ask for confirmation on multi-user install without systemd
installer: ask for confirmation on multi-user install without systemd

On Linux a user can go through all the way through the multi-user install
and find out at the end that they now have to manually configure their
init system to launch the nix daemon.

I suspect that for a significant number of users this is not
what they wanted. They might prefer a single-user install.
Now they have to manually uninstall nix before they can
go through the single-user install.

This introduces a confirmation dialog before the install
in that specific situation to make sure that they want to proceed.

See also: https://github.com/NixOS/nix/issues/4999#issuecomment-1064188080
This closes #4999 but rejecting it and closing that issue anyways
would also be valid.

NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix