core/unit: allow overriding an ongoing freeze operation
    
    Sometimes a freeze operation can hang due to the presence of kernel
    threads inside the unit cgroup (e.g. QEMU-KVM). This ensures that the
    ThawUnit operation invoked by systemd-sleep at wakeup always thaws the
    unit.
    
    (cherry picked from commit 3d19e122cfe341b28dfcb58f1aac829c122da569)