Report container exit status to monitord

When managing containers, I need to take action based on container
exit status. For instance, if it exited abnormally (status!=0), I
sometime want to respawn it automatically. Or, when invoking
`lxc-stop` I want to know if it terminated gracefully (ie on `SIGTERM`)
or on `SIGKILL` after a timeout.

This patch adds a new message type `lxc_msg_exit_code,` to preserve
ABI. It sends the raw status code as returned by `waitpid` so that
listening application may want to apply `WEXITSTATUS` before. This is
what `lxc-monitor` does.

Signed-off-by: Jean-Tiare LE BIGOT <jean-tiare.le-bigot@ovh.net>