python-paste-script: insecure root GID accessible files

A security flaw was found in the way Paster, a pluggable command-line frontend, when started as root (for example to have access to privileged port) to serve a web based application, performed privileges dropping upon startup (supplementary groups were not dropped properly regardless of the UID, GID specified in the .ini configuration file or in the --user and --group CL arguments). A remote attacker could use this flaw for example to read / write root GID accessible files, if the particular web application provided remote means for local file manipulation.