zshrc: export SHELL only if unset, don't hardcode path for zsh binary

On e.g. FreeBSD zsh isnt' necessarily available as /bin/zsh, so
let's make that more flexible. Also while at it export SHELL only
if the variable is unset yet, since setting SHELL should be
something the login system care about.

Acked-by: Frank Terbeck <ft@grml.org>