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/options.lua

@@ -812,7 +812,7 @@ return {
       vi_def=true,
       redraw={'all_windows'},
       varname='p_fcs',
-      defaults={if_true={vi="vert:|,fold:-"}}
+      defaults={if_true={vi="vert:│,fold:·"}}
     },
     {
       full_name='fixendofline', abbreviation='fixeol',