This patch introduces the --clear-env and --keep-env options for
lxc-attach, that allows the user to specify whether the environment
should be passed on inside the container or not.

This is to be expanded upon in later versions, this patch only
introduces the most basic functionality.
Signed-off-by: Christian Seiler <christian@iwakd.de>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>