Avoid ServeApi race condition

If job "acceptconnections" is called before "serveapi" the API Accept()
method will hang forever waiting for activation.  This is due to the fact
that when "acceptconnections" ran the activation channel was nil.

Signed-off-by: Darren Shepherd <darren@rancher.com>
(cherry picked from commit 8f6a14452dfd88aedc8ac9577a98c38a555baadc)

Docker-DCO-1.1-Signed-off-by: Jessie Frazelle <princess@docker.com> (github: jfrazelle)

Docker-DCO-1.1-Signed-off-by: Jessie Frazelle <hugs@docker.com> (github: jfrazelle)

api/server/server.go

@@ -38,7 +38,7 @@ import (
 )
 
 var (
-	activationLock chan struct{}
+	activationLock chan struct{} = make(chan struct{})
 )
 
 type HttpServer struct {
@@ -1578,7 +1578,6 @@ func ServeApi(job *engine.Job) engine.Status {
 		protoAddrs = job.Args
 		chErrors   = make(chan error, len(protoAddrs))
 	)
-	activationLock = make(chan struct{})
 
 	for _, protoAddr := range protoAddrs {
 		protoAddrParts := strings.SplitN(protoAddr, "://", 2)