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: NixOS#4999 (comment)
This closes NixOS#4999 but rejecting it and closing that issue anyways
would also be valid.

scripts/install-multi-user.sh

@@ -423,6 +423,18 @@ EOF
         fi
     done
 
+    if [ "$(uname -s)" = "Linux" ] && [ ! -e /run/systemd/system ]; then
+        warning <<EOF
+We did not detect systemd on your system. With a multi-user install
+without systemd you will have to manually configure your init system to
+launch the Nix daemon after installation.
+EOF
+        if ! ui_confirm "Do you want to proceed with a multi-user installation?"; then
+            failure <<EOF
+You have aborted the installation.
+EOF
+        fi
+    fi
 }
 
 setup_report() {