Add watchdog

We enabled RuntimeWatchdogSec=5min in !304 (merged), but systemd is apparently still "pinging" the watchdog if the system is trashing heavily.

Perhaps we should switch to watchdog^AUR? or something more configurable? or https://github.com/troglobit/watchdogd (I also created a issue for PSI support: https://github.com/troglobit/watchdogd/issues/25)