lxc-attach: Clear environment and set container=lxc

The child process's environment should be manipulated the same way
by lxc-attach as it would be by lxc-start or lxc-execute.

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>