Make blocking channel for StartTransientUnit buffered

So that, if a timeout happens and we decide to stop blocking on the
operation, the writer will not block when they try to report the result
of the operation.

This should address Issue opencontainers#1780 and it's a follow up for PR opencontainers#1683,
PR opencontainers#1754 and PR opencontainers#1772.

libcontainer/cgroups/systemd/apply_systemd.go

@@ -320,7 +320,7 @@ func (m *Manager) Apply(pid int) error {
 		}
 	}
 
-	statusChan := make(chan string)
+	statusChan := make(chan string, 1)
 	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, statusChan); err == nil {
 		select {
 		case <-statusChan: