zshrc: Make /etc/zsh/functions.d recursive

zshrc: Make /etc/zsh/functions.d recursive

That way, we can drop whole subsystems into there, if we want to, like:

    /etc/zsh/functions.d/VCS_Info/...

This change also makes the code more backwards compatible.
zrcautoload does not need the -U switch. It does that automatically
depending on which zsh version is running.