Add system roll-back and switch-generation commands

[cmmarusich]

Hi,

This patch series adds 'guix system roll-back' and 'guix system
switch-generation' commands.  It should apply cleanly to the current master
branch.

I've tested the patches manually on a VM with a single disk, and a VM with
multiple disks (with the store on a separate disk).  Two of the "check-system"
tests fail (nss-mdns and encrypted-root-os), but those are failing on the
current master branch for me anyway, and I can't recall ever seeing them pass,
so I have no reason to suspect that my changes are causing the failures.

I look forward to your feedback.

Thank you,
Chris