Bookmarked directory with a space in the name

This may be the result of a bug in vim and neovim, but it appears that passing an escaped directory path to `isdirectory` results in a false negative. This prevents a bookmark such as `/Path/To Directory` from changing directories, though it will still open the directory in netrw.

I've done the following debugging:

```
let dir='/path/with space'
echo isdirectory(dir) => 1
echo isdirectory(fnameescape(dir)) => 0
```

This effects `startify` because the escaped path is passed into `s:check_user_options(path)` and the checks for `isdirectory` there prevent it from ever attempting to change directory.

Inconsistently, `execute 'lcd' dir` or `execute 'lcd' fnameescape(dir)` both work just fine. It only appears to be an issue with passing as a parameter. I also don't see a method for unescaping the path. I thought about using substitution, but that would break for Windows paths.

Not sure if there's another work around here besides renaming my directories.

Start by tracing the bookmark path through s:check_user_options(path), focusing on how the escaped directory is passed to isdirectory. Reproduce the issue with a bookmarked path containing spaces in Vim or Neovim, and consider Windows paths while checking the existing directory-change behavior. Done means such bookmarks pass validation and change directories without breaking existing paths.

Written by the indexing model from the issue text.