This patch implements the extra_env and extra_keep options of
lxc_attach_set_environment.

The Python implementation, the C container API and the lxc-attach
utility are able to utilize this feature; lxc-attach has gained two new
command line options for this.
Signed-off-by: Christian Seiler <christian@iwakd.de>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>