Add a check that $CONFIG_DIR is an absolute path.

This caused me problems when trying to run setup_fns.sh manually.

Auditors: chris

Test plan:
Ran this:

    % sh shared/setup_fns.sh
    You must set CONFIG_DIR before sourcing setup_fns.sh

    % env CONFIG_DIR=a sh shared/setup_fns.sh
    0
    CONFIG_DIR must be an absolute path, not 'a'.

    % env CONFIG_DIR=/ sh shared/setup_fns.sh
    1