lxc: introduce lxc-kill command (v4)

lxc: introduce lxc-kill command (v4)

lxc-kill send a signal to the process 1 of the container.

If this command is used on an application container ran by
lxc-execute, the lxc-init will receive the signal and will forward it to
the process 2 which is the command specified in the command line.

Signed-off-by: Greg Kurz <gkurz@fr.ibm.com>
Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>