shutdown: Rework API and lxc-stop

With this change, shutdown() will no longer call stop() after the
timeout, instead it'll just return false and it's up to the caller to
then call stop() if appropriate.

This also updates the bindings, tests and other scripts.

lxc-stop is then updated to do proper option checking and use shutdown,
stop or reboot as appropriate.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>