systemctl: be nice to users and give hint how to specify "-.mount"

systemctl: be nice to users and give hint how to specify "-.mount"

https://bugzilla.redhat.com/show_bug.cgi?id=1656639
Using "--" is a trick that is hard to discover. Let's give users a hint:

$ build/systemctl status -.service
  build/systemctl: invalid option -- '.'
  Hint: to specify units starting with a dash, use "--":
        build/systemctl [OPTIONS...] {COMMAND} -- -.service ...

I use program_invocation_name because that's what getopt seems to use.
"::" is used in the option string so that getopt doesn't complain about
a missing argument in case somebody passes "-." as the argument. After all
"." is not a real option.