The previous to wait-types (NM_SHUTDOWN_WAIT_TYPE_OBJECT and
NM_SHUTDOWN_WAIT_TYPE_CANCELLABLE) both required a GObject/GCancellable,
and the shutdown was automatically unblocked when the object got
destroyed.

Add another wait type NM_SHUTDOWN_WAIT_TYPE_HANDLE, which does not take
an object to wait. Instead, shutdown is indefinitely blocked, until the
user unregisters the handle again. While other wait-types allow to
ignore the handle, this wait-type only makes sense if the user keeps
track of the handle.