ex_getln: add secret charm

Opt in to this secret world using

    set wildchar=0
    " already the default, but remove if non-zero existing config:
    set wildcharm=0

now you can map 'wildmode' just like any mode:

    cnoremap <tab> <c-z>
    function! Spacey()
      return getcmdline()[-1:] == "/" ? "\<bs>" : ""
    endfunc
    cnoremap <expr> / wildmenumode() ? Spacey()."/<c-z>" : "/"

Possibly asked questions:

What about backwards compatibility?
====

Just do nothing and your existing 'wildchar' and 'wildcharm' will keep working.

Doesn't `<c-z>` mean suspend?
====

Not in cmdline mode. If it would then the recommended wildcharm would not
have been `<c-z>` to start with.

My config relies on `:<c-z>` being a synonym to `:<nop>`!
====
just no.

test/functional/helpers.lua

@@ -442,6 +442,7 @@ function module.new_argv(...)
         'NVIM_LOG_FILE',
         'NVIM_RPLUGIN_MANIFEST',
         'GCOV_ERROR_FILE',
+        'XDG_DATA_DIRS',
         'TMPDIR',
       }) do
         if not env_tbl[k] then