zsh: fixup env vars and update a comment

- removed graphical env vars as they aren't really doing anything. would be
  useful for unmanaged invocations of xorg/wayland, but I don't do those
  anymore. if graphical configuration is ever needed it should be added to site
  local rc anyway since it relies heavily on underlying system configuration.
- scoped vars inside functions instead of unsetting.
- scoped some arithmetic vars inside _sev_setpath that were not being unset.
- added /opt/<provider>/<package>/bin and /opt/<package>/bin to path per FHS.
- be more aggressive in unsetting GNUPGHOME after forwarding.
- updated a gpg comment for clarification while there.