zsh: Remove gnuplot stuff from .zlogin.

I haven't used gnuplot in ages.
zsh: Consolidate startup scripts into one folder.

The whole "interactive"/"login" split was going to get weird when
I added one for .zshenv. We'll just dump them all in one place instead
and distinguish them by filename suffix.
zsh: Make .zshenv source auxiliary login scripts.

You know the drill by now.
mp: Break out zsh customization into new topic folder.