Fix highlighting of full-width space and disable it.

- Fix highlighting of full-width space.
  Sometimes (for example, when window is splitted) highlight cancelled because autocmd was not used before.
  Now full-width space is always highlighted.
- Disabled highlighting of full-width space.
  I use Ricty font which enables to distinguish half-width space and
  full-width space.
  Of course I can't use the font when I am on virtual console, but then
  I can't edit text with full-width character, so it is not a problem.
  So I have no reason to highlight full-width space ;-P