Make fullscreen with margins

[Christopher Dimech <dimech@gmx.com>]

I have the following code for getting to full screen Have made buffer-margins
that sets a margin on either side of buffer.  Bet I would like that the
margins are only set when there is only one buffer in the frame.

(defun fullscr-cycle ()
   "Toggles Fullscreen Mode"
   (interactive)
   (if (eq (frame-parameter nil 'fullscreen) 'fullboth)
      (progn (buffer-margins 0)
             (disable-fullscr))
      (progn (buffer-margins)  ! Defaults margins to width of 3
             (enable-fullscr)) ))