This is none of option.c business to create *possibly unneeded* **default**
directories **before** user specified where he actually wants to place the
files.
- Add functions that are able to query XDG.
- Replace defaults for
- &runtimepath. Does not follow #78.
- &viewdir.
- &undodir.
- &directory.
- &backupdir. Does not follow #78.
- vimrc location.
- Remove user vimrc file line from :version message.