Fix race against systemd

- T0: runc triggers a systemd unit creation asynchronously from [here](https://github.com/opencontainers/runc/blob/master/libcontainer/cgroups/systemd/apply_systemd.go#L298)
- T1: runc then moves ahead and starts creating cgroup paths(.scope directories), [here](https://github.com/opencontainers/runc/blob/master/libcontainer/cgroups/systemd/apply_systemd.go#L348). Kernel creates .scope directory and cgroup.procs file(along with other default files) in the directory automatically, in an atomic manner.
- T3: systemd execution thread which was invoked at time `T0`, is still in the process of unit creation. systemd also trying to create cgroup paths and deletes the `.scope` directory which is created at time `T1` by runc from [here](https://github.com/systemd/systemd/blob/v219/src/shared/cgroup-util.c#L1630) in the code

Signed-off-by: vikaschoudhary16 <[email protected]>

Author: vikaschoudhary16

libcontainer/cgroups/systemd/apply_systemd.go

@@ -295,10 +295,13 @@ func (m *Manager) Apply(pid int) error {
 		}
 	}
 
-	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, nil); err != nil && !isUnitExists(err) {
+	statusChan := make(chan string)
+	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, statusChan); err != nil && !isUnitExists(err) {
 		return err
 	}
 
+	<-statusChan
+
 	if err := joinCgroups(c, pid); err != nil {
 		return err
 	}