portablectl: add --force attach/detach

portablectl: add --force attach/detach

Allows to skip check that ensures units must not be running.

I have a use case that would use reattach, except the orchestrator
is using a non-standard versioning scheme, so image matching cannot
work. As a workaround, need to be able to detach and then attach
manually, without stopping the units to avoid extended downtimes
and loss of FD store.