session: Clear environment when running cockpit-session setuid

Environment variables can affect all sorts of things like LD_PRELOAD,
PAM module behavior, and so on. When cockpit-session is running as
a setuid process, clear the environment completely.

This means that for debugging using G_DEBUG or G_MESSAGES_DEBUG
cockpit-ws must be running as root. Update HACKING.md to reflect this.

Closes #1123
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>