Fix ZSH integration part of Xsession script

This is a long-time issue for Kubuntu it seems.

See https://bugs.launchpad.net/ubuntu/+source/kdebase/+bug/13965 (KDM
contains the same code, and people also had issues logging in with ZSH
here). The bug report is from 2005 ;)
Merge pull request #287 from krf/make-zsh-integration-work

Fix ZSH integration part of Xsession script