guix system: Add "--file-system-type" option.

* guix/scripts/system.scm (process-action): Pass file-system-type to ...
(perform-action): ... here.  Add new keyword argument.  Pass new value to ...
(system-derivation-for-action): ... here.  Add new keyword argument.
Pass new value to system-disk-image.
* doc/guix.texi (disk-image): Document new option.