defaults: 'showcmd', 'belloff', 'ruler'

- Vim "unix default" of 'noshowcmd' is serving few users. And it's
  inconsistent.
- 'ruler' and 'belloff=all' improve the out-of-the-box experience.
- Continue to use 'noshowcmd' and 'noruler' by default in the functional
  tests to keep them fast.

TODO: Add a "disable slow stuff" command or mapping to address the
use-case of a very slow terminal connection.

test/functional/ui/screen_basic_spec.lua

@@ -9,7 +9,7 @@ local eval = helpers.eval
 describe('Initial screen', function()
   local screen
   local nvim_argv = {helpers.nvim_prog, '-u', 'NONE', '-i', 'NONE', '-N',
-                     '--cmd', 'set shortmess+=I background=light noswapfile',
+                     '--cmd', 'set shortmess+=I background=light noswapfile belloff= noshowcmd noruler',
                      '--embed'}
 
   before_each(function()