Fix compliance with POSIX shells

cnf.sh breaks GDM (and maybe other DMs) when Dash is used for /bin/sh.
It is usually sourced from /etc/gdm/Xsession -> /etc/profile -> /etc/profile.d/cnf.sh.
The breakage is due to cnf.sh using a "function" bashism: https://wiki.ubuntu.com/DashAsBinSh#function

Simply removing the "function" keyword fixes the problem.
No regressions are introduced.