(struct buffer): Rename members measured in columns:

(struct buffer): Rename members measured in columns:
left_margin_width to left_margin_cols, right_margin_width to
right_margin_cols.  All uses changed.
New members left_fringe_width, right_fringe_width,
fringes_outside_margins for per-buffer fringe settings.
New members scroll_bar_width and vertical_scroll_bar_type for
per-buffer scroll bar settings.