watchdog triggered auto-rollback

[Nathan Dehnel]

>Would others find this useful?
I would 100% use this.

>Where in the stack would this be solved?
I think there's two places for rollbacks with two different purposes

GRUB: https://www.gnu.org/software/grub/manual/grub/html_node/fallback.html
GRUB supports falling back to another boot entry if the machine fails
to boot. This could be integrated with guix so GRUB falls back to a
previous guix system generation. This covers the case of "we can't
start a watchdog service because the system won't boot".

SSH watchdog: a shepherd service that tests SSH connectivity, and then
executes "guix system roll-back && reboot". SSH access is a rough
approximation for "the system is working", as kernel, init, and all
manner of networking services, DHCP, DNS, VPN, etc. must work for SSH
to work. And if SSH works then it provides a means for a user to fix
their system anyways.