Dear GUIX maintainers,

Current the GUIX SD boot process does not allow resuming from sleep,
even thought sleep options are available through loginctl, eg:

loginctl hybrid-sleep
loginctl hibernate

This is a very important feature for people like me using GUIX SD on a
laptop (yes, it is possible, mine is a corebooted X230 running
linux-libre!)

This patch is based on a patch floating around. The core functionality
has been isolated, the resume function isolated, the patch rebased and
tested. I'm not taking credit for it, even though tracing the exact
origin is hard.

The resume hook is called if the resume= kernel argument is provided,
which one can do during system configuration.

My scheme level is zero, so please bear with me. In particular, some
conditionals could maybe be moved within the function, or the function
itself called within some already-available hooks. Also it is not clear
if the commit log is adequate for such a change.

Please let me know how to improve this and get this merged; I can also
write some documentation (probably once the mechanism is in place) to
explain how the feature can be used.

Kind regards,
Jean-Baptiste