systemctl: simplify halt_main()

systemctl: simplify halt_main()

The code at this point is not able to tell whether it was called as
halt/poweroff/reboot or shutdown with time "now".
The code also takes a shortcut to skip logind if called as root.
That however means asking shutdown for immediate action won't trigger a
wall message.
As per https://github.com/systemd/systemd/issues/8424#issuecomment-374677315
all commands should trigger a wall message.
That simplifies the code as we can try logind first always.