defaults: systemd: Force poweroff instead of halt

Some systems power off after executing "halt". Users with a habit of "halt" usage may end up surprised when the Librem 5 appears completely off after using it, but actually is not - after halting, the SoC remains on, consumes about 1W of power and does not respond to any input (including low battery level!) until the power button is held for ~5 seconds or power is cut.

There's little reason to ever halt a mobile phone without powering it off, so don't let the users shoot themselves in their foot (especially given that this can easily end up with battery's undercharge protection circuit being involved) and force poweroff whenever halt is invoked.