add a start-host hook (v2)

add a start-host hook (v2)

This should satisfy several use cases.  The one I tested for was CNI.
I replaced the network configuration in a root owned container with:

lxc.net.0.type = empty
lxc.hook.start-host = /bin/lxc-start-netns

where /bin/lxc-start-netns contained:

=================================

echo "starting" > /tmp/debug
ip link add host1 type veth peer name peer1
ip link set host1 master lxcbr0
ip link set host1 up
ip link set peer1 netns "${LXC_PID}"
=================================

The nic 'peer1' was placed into the container as expected.

For this to work, we pass the container init's pid as LXC_PID in
an environment variable, since lxc-info cannot work at that point.

Signed-off-by: Serge Hallyn <shallyn@cisco.com>