guix: system: Make disk-image root file system non-volatile by default.

guix: system: Make disk-image root file system non-volatile by default.

And add a new '--volatile' option to have it volatile otherwise.

* guix/scripts/system.scm (%options)[volatile-root?]: New boolean option.
(%default-options): Set its default value to #f.
(show-help): Add help doc.
* guix/scripts/system.scm (perform-action): Propagate option...
(system-derivation-for-action): ...here.  Use it to set the volatile-root?
field of the image object passed to SYSTEM-IMAGE.
* doc/guix.texi (Invoking guix system): Document it.