Improve and fix zsh completion
This fixes some aspects of the zsh completion. Option exclusion lists wrongly included things like =DIR on the end of the option which prevents the exclusions from working. I tweaked the descriptions to follow the zsh convention of starting with a lowercase verb in imperative mood form. Return status was not correct in some obscure cases which would break approximate completion of options. Also the context variable was ignored which it shouldn't have been. Using curcontext and passing -C to _arguments is simpler so this does that.
Note that we have removed the _zathura function that was recently contributed to zsh itself. It is better for projects to include their own functions.