Better fix for .zshenv and minor corrections for other zsh files (#100)

* zsh: better fix for .zshenv

* zsh: add export keyword to variables

* zsh: variables => variable

Only one environment variable is suggested in every help message

* zsh: remove leading spaces and redundant newlines

* zsh: .zshenv: remove note about sudo privileges

Not needed, since it is obvious

* zsh: clarify where to move files

* zsh: quote all variables