The previous lxc-shutdown change replaced 'kill SIG<name>' by
'kill -s SIG<name>'. Although this works with busybox where it was tested,
this doesn't actually work with all kill implementations. Some requiring just
the signal name without the prefix.

This changes "-s SIG<name>" by just "-s <name>". Tested with busybox and
standard kill.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>