defaults: 'fillchars'

Most fonts should have these by now. Both are a significant visual
improvement.

- Vertical connecting bar `│` is used by tmux, pstree, Windows 7 cmd.exe
  and nvim-qt.exe.
- Middle dot `·` works on Windows 7 cmd.exe, nvim-qt.exe.

For reference: tmux uses these chars to draw lines: │ ├ ─

src/nvim/screen.c

@@ -7089,7 +7089,7 @@ static int fillchar_vsep(win_T *wp, int *attr)
 {
   *attr = win_hl_attr(wp, HLF_C);
   if (*attr == 0 && fill_vert == ' ') {
-    return '|';
+    return 9474;  // default: "│"
   } else {
     return fill_vert;
   }