The minimum number of columns of any window.
The value has to accommodate margins, fringes, or scrollbars if
present. A value less than `window-safe-min-width' is ignored.
The value of this variable is honored when windows are resized or
split.
Applications should never rebind this variable. To resize a
window to a width less than the one specified here, an
application should instead call `window-resize' with a non-nil
IGNORE argument. In order to have `split-window' make a window
narrower, explicitly specify the SIZE argument of that function.