system: Rename 'disk-image' command 'image'.

* guix/scripts/system.scm (system-derivation-for-action): Rename 'disk-image'
command 'image'. Warn when using the now deprecated 'disk-image' command.
(show-help): Adapt accordingly.
(guix-system): Ditto.
* tests/guix-system.sh: Ditto.
* gnu/system/examples/bare-hurd.tmpl: Ditto.
* doc/guix.texi (Building the Installation Image,
Building the Installation Image for ARM Boards,
Invoking guix pack,
Invoking guix system): Adapt documentation.