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/options/defaults_spec.lua

@@ -11,9 +11,7 @@ local neq = helpers.neq
 
 local function init_session(...)
   local args = { helpers.nvim_prog, '-i', 'NONE', '--embed',
-    '--cmd', 'set shortmess+=I background=light noswapfile noautoindent',
-    '--cmd', 'set laststatus=1 undodir=. directory=. viewdir=. backupdir=.'
-    }
+                 '--cmd', helpers.nvim_set }
   for _, v in ipairs({...}) do
     table.insert(args, v)
   end