fix(api): serve until the "acceptconnections" job

This fixes a bug that I encountered when using socket activation with
docker 0.8.1. When running the first `docker run` it would return:
"create: command not found".

The root cause was the socket activation code path was starting to
listen before the "initserver" job had finished. This meant that the
"create" handler hand't been registered yet leading to the command not
found error.

In log format it looks like this:

```
[/var/lib/docker|9d2e78e9] +job initserver()
2014/03/01 04:05:35 Listening for HTTP on fd ()
[/var/lib/docker|0d71c177] +job create()
create: command not found
[/var/lib/docker|0d71c177] -job create()
[/var/lib/docker|0d71c177] +job acceptconnections()
[/var/lib/docker|0d71c177] -job initserver() = OK (0)
```

To fix the issue select on the activationLock and block until the
"acceptconnections" job has ran.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)

api/server.go

@@ -1079,6 +1079,11 @@ func ServeFd(addr string, handle http.Handler) error {
 
 	chErrors := make(chan error, len(ls))
 
+	// We don't want to start serving on these sockets until the
+	// "initserver" job has completed. Otherwise required handlers
+	// won't be ready.
+	<-activationLock
+
 	// Since ListenFD will return one or more sockets we have
 	// to create a go func to spawn off multiple serves
 	for i := range ls {