As I started to type on the international US layout I don't need any
extra bindings for special characters in vim.
Move the update of `spellfile` into xdg.vim, as it was the only
remaining body of `NewSpellLang()` and fits thematically.
Fix bug that when the XDG Base Directory variables are not set on
startup a directory "~" is created in $HOME instead of falling back to
the default values.
When TERM is not xterm-256color (for example in st or tmux) t_8f and
t_8b have to be set manually for colors to work.
Add XDG_CONFIG_HOME/vim to packpath so that plugins work.