Fixes acassen/keepalived#104 and acassen/keepalived#114

Each time check_child recieves SIGHUP two netlink channels are being opened in kernel_netlink_init(): nl_cmd and nl_kernel. nl_kernel is being attached to a kernel_netlink thread and is being closed when thread_destroy_master() is called from reload_check_thread(). nl_cmd channel has no thread attached, therefore it leaks.