`~/.zshenv' is executed for _every_ instance of `zsh', so we should
limit the work done there to the absolute minimum. For me, that
currently means only making sure `$ZDOTDIR' is set so that login (via
`.zprofile') and interactive (via `.zshrc') shells can be configured
appropriately.
Neovim always uses `xclip` as the clipboard provider if DISPLAY is set.
Therefore, if we set it unconditionally, `xclip` hangs while trying to
reach the server if this hasn't been started, causing all sorts of
chaos.
