zshrc: drop setting TZ via /etc/timezone

This shouldn't be part of zshrc.
zshrc: do not source /etc/sysconfig/keyboard

There shouldn't be any need for doing this here.
zshrc: check for DIRSTACK{SIZE,FILE} in chpwd()

If the environment variables are unset chpwd emmits an
error message, so check before using.

Thanks: Frank Terbeck <ft@grml.org> for review