guix system: 'guix system init' installs GRUB by default.

* guix/scripts/system.scm (install): Add #:grub?, #:grub.cfg, and
  #:device parameters; honor them.
  (show-help): Document '--no-grub'.
  (%options): Add '--no-grub'.
  (%default-options): Add 'install-grub?'.
  (guix-system): Honor 'install-grub?' option from OPTS.  Adjust
  'install' call accordingly.
* doc/guix.texi (Invoking guix system): Document '--no-grub'.