Improve feedback for startup

Improve feedback for startup

Since it was necessary to switch to 'Type=Simple' in the systemd
service, see 545d6f0a13ac2bf3a8d3f224c19c0e0def12116d,
'systemctl start' would not wait for the 'lxc-start' command anymore.
Thus every container start was reported as a success and the 'post-start'
hook would trigger immediately after the 'systemctl start' command.

Use the monitor socket to get the necessary information and detect
startup failure, and only run the 'post-start' hookscript after
the container is effectively running. If something goes wrong
with the monitor socket, for example if lxc-monitord is not running,
fall back to the old behavior.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>