MFC r270280 by dumbbell: vt(4): Pause the flush timer while swithing window

  This fixes bad looking refresh when switching window: squares instead
  of text, flashing screen, and so on. In the worst case, vt_flush() came
  at a very inappropriate timing and the screen was not refreshed at all
  (leaving squares all over the place).

  This doesn't fix the flickering of the screen with vt_vga, because the
  sync signal is temporarily stopped and the video memory is cleared.

Sponsored by:	The FreeBSD Foundation