Follow-up: Add system roll-back and switch-generation commands

[cmmarusich]

This patch series builds on 1ef8b72a7f87afe7cffe52393d99e1b14e4770e1 to add
two new 'guix system' actions: roll-back and switch-generation.

I've verified that both 'make check' and 'make check-system' pass (with the
exception of the system tests 'nss-mdns' and 'encrypted-root-os', which both
fail on my system even without my patches).  I've manually verified the
following for a VM with a single disk and single partition, as well as for a
VM with multiple disks and the store on a separate partition:

* The grub.cfg file that 'roll-back' and 'switch-generation' generates looks
  correct and is correctly registered as a GC root.

* The system can boot after 'roll-back' and 'switch-generation', the default
  entry is correct, and the resources necessary for the GRUB theme are loaded
  without error.

* Any attempt to switch to a nonexistent generation fail, regardless
  of whether a relative or absolute generation spec is used.

* The existing 'list-generations' command continues to show the
  generations correctly, even after switching generations.

Hopefully everything is OK this time!  Thank you for taking the time to review
it.

--
Chris