add lxc.haltsignal for soft shutdown

- use this in the busybox template since busybox's init expects
  to receive SIGUSR1 to halt

- fix lxc.stopsignal to be output by write_config so lxcapi_clone()
  and lxcapi_save_config() will output it

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>