ZSH fixes (2017.03.08.)

As it turns out, the previous update of setup.sh.in made it unusable for ZSH.

I tried a number of different incantations, and finally settled on this. This should be calling the builtin cd and pwd commands even if the user introduced aliases for them. And it definitely works as it should with both BASH and ZSH.

If this formalism still doesn't work in somebody's non-standard environment, then it shouldn't be on this piece of code to fix this anymore...