systemctl: print a notice when set-default is not effective

$ sudo ln -svf multi-user.target /run/systemd/generator.early/default.target
'/run/systemd/generator.early/default.target' -> 'multi-user.target'
$ sudo build/systemctl set-default --root=/ sysinit.target
Removed /etc/systemd/system/default.target.
Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/sysinit.target.
Note: "multi-user.target" is the default unit (possibly a runtime override).

The output is not super informative, but it should be enough to point the user in
the right direction.

Fixes #3645.