system: Support the --root option in 'guix system'.

Fixes <https://bugs.gnu.org/26271

>.

* guix/scripts/system.scm (perform-action): Add #:gc-root parameter and
honor it.
(show-help): Document the --root option.
(%options): Add 'root'.
(process-action): Pass 'root' option to perform-action as #:gc-root.
* doc/guix.texi (Invoking guix system): Document '--root'.

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