stop: add nolock option

If the system gets into a bad state, it may become impossible to get
the lxc container locks.  We should still be able to stop containers
in that case.  Add a -L/--nolock option to specify this behavior.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>