wait-online: exit with success when all interfaces are ready or unmanaged

wait-online: exit with success when all interfaces are ready or unmanaged

Previously, when neither '--any' nor '--interface' options specified,
at least one of the interfaces must be in configured state.
This patch makes wait-online exit with success even if all interfaces
are in unmanaged state.

This may break backward compatibility in a rare situation. But in most
cases, this typically not change anything, as at least one interface
is managed by networkd.service when it is enabled.

This is mostly for making wait-online gracefully handle the case that
networkd.service and wait-online.service are enabled by mistake. In such
situation, all interfaces are typically not managed.

Fixes #25813.