system: Add 'guix system' actions: switch-generation and roll-back.

* guix/scripts/system.scm (roll-back-system, switch-to-system-generation): new
actions.
(reinstall-grub): New procedure, used by switch-to-system-generation.
(show-help, process-command, guix-system): Honor the new actions.
* doc/guix.texi (Invoking guix system) <switch-generation, roll-back>: Add the
new actions.
<reconfigure>: In the footnote, mention that the new actions also only work on
GuixSD.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>