code cleanup, add su support

cleanup renames some internal vars, moves SSH/GPG setup back to .zshenv,
and upgrades SSH setup to give more information about what it is doing.

su support will allow you to switch to another user with a non-login
shell and, as long as they are using zsh with the same dotfiles, you
will get a proper environment set up as if it were a login shell; this
is a lot easier and more comfortable than making an alias to or manually
passing SSH_AUTH_AGENT and other vars via sudo... for now.

an attempt should be made to make super user switching more portable by
adding support for doas and bog-standard su in aliases and elsewhere.