* rename lxc.haltsignal to lxc.signal.halt
* rename lxc.rebootsignal to lxc.signal.reboot
* rename lxc.stopsignal to lxc.signal.stop

the legacy keys will be kept around until LXC 3.0 and then will be
removed.
Signed-off-by: 0x0916 <w@laoqinren.net>