guix system: Add 'init' sub-command.

* guix/scripts/system.scm (install): New procedure.
  (guix-system)[parse-option]: Remove check for extraneous arguments.
  [match-pair, option-arguments]: New procedures.
  Use 'option-arguments'.  Honor 'init'.
  (show-help): Document 'init'.
* doc/guix.texi (Invoking guix system): Document 'init'.