add environment variable whitelist to debuginfo install wrapper - closes 692

The install-debuginfo C wrapper which is run by a non-root user
clears the whole environment. However, since the installer
asks for user input by the use of an environment variable,
we have to create a whitelist of variables to keep when doing
the environment purge.

Signed-off-by: Petr Kubat <pkubat@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>