Integrate zzz, xscreensaver and rc.suspend

I just installed helloSystem on a real hardware, finally, so expect a lot of bug reports in the coming days :P

Hardware: Lenovo ThinkPad T480s

I did `sudo zzz` in the terminal, and it all worked fine. So a proper way to get this integrated is the following.

1) Add a "suspend/sleep" button in the power options (which, btw, the menu makes no sense, since selecting Shut Down / Restart / Log Out brings the same dialog)
2) When the hardware is about to suspend, FreeBSD runs `/etc/rc.suspend`, we can add `xscreensaver-command -lock` in there, so it locks the screen and sleeps
3) The hardware is back on! now there's a screensaver, very good security feature!

I can see that `hw.acpi.lid_switch_state` is set to `S3` already, so it will work with the lid as well!