setup-timezone: make -z backwards compatible
Since -z is still present in getopts parsing, it looks like the removal
of the option was supposed to be backwards compatible. Fix the code to
make it so.
Fixes: 31e63870 ("setup-timezone: fix usage, drop -z option from help")
@ncopa: we noticed this because pmbootstrap had to be adjusted. This patch might be useful for other people who then don't need to adjust their scripts.