Add option to rename container to lxc-clone

Add option to rename container to lxc-clone

This commit adds an -R, --rename option to lxc-clone to rename a container. As
c->rename calls do_lxcapi_rename() which in turn calls do_lxcapi_clone() it
seemed best to implement it in lxc-clone rather than lxc-snapshot which also
calls do_lxcapi_clone(). Some additional unification regarding the usage of
return vs exit() in main() was done.

Signed-off-by: Christian Brauner <christianvanbrauner@gmail.com>