Allow service to also disconnect at shutdown

[dschwartz783]

Added to keep service running in background, so that nvme can be disconnected at shutdown.

Currently, at least on Ubuntu, a variety of services hang due to the nvme device files being tied to active connections, which no longer work once the network has been brought down. This fixes that. Connections will be terminated before passing network-online.target.

[tbzatek]

Well this needs to exclude connections that provide the system itself, e.g. access to rootfs, /boot, /usr (if separate), /var etc. Including any connection done within initramfs. And not only that, for NBFT connections additional device paths may have been connected during the system runtime and the original device paths may get disconnected.

If the application strings / context would exclude such connections from disconnect-all, we'd need to consistently mark all such connections. Similar to excluding network interfaces being torn down if they provide access to rootfs connections.

[tbzatek]

See also #2913