Move supervisor to it's own package

It allows to keep main namespace cleaner

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

supervisor/signal.go

@@ -0,0 +1,22 @@
+package supervisor
+
+type SignalEvent struct {
+	s *Supervisor
+}
+
+func (h *SignalEvent) Handle(e *Event) error {
+	i, ok := h.s.containers[e.ID]
+	if !ok {
+		return ErrContainerNotFound
+	}
+	processes, err := i.container.Processes()
+	if err != nil {
+		return err
+	}
+	for _, p := range processes {
+		if pid, err := p.Pid(); err == nil && pid == e.Pid {
+			return p.Signal(e.Signal)
+		}
+	}
+	return ErrProcessNotFound
+}