nspawn: give better message when invoked as non-root without arguments

nspawn: give better message when invoked as non-root without arguments

When invoked as non-root, we would suggest re-running as root without any
further hint. But this immediately spawns a machine from the local directory,
which can be rather surprising. So let's give a better hint.

(In general, I don't think commandline programs should do "significant" things
when invoked without any arguments. In this regard it would be better if
systemd-nspawn would not spawn a machine from the current directory if called
with no arguments and at least "-D ." would be required.)