Be more strict about file closing in bind

This eliminates the race condition mentioned in a54c913a by forbidding
duplicate binds to the same socket (well, at least in the sense that
attempting to do so will *always* result in an error instead of
nondeterministically resulting in an error).

bind/bind.go

@@ -75,6 +75,7 @@ func listenTo(bind string) (net.Listener, error) {
 				bind, err)
 		}
 		f := os.NewFile(uintptr(fd), bind)
+		defer f.Close()
 		return net.FileListener(f)
 	} else if strings.HasPrefix(bind, "einhorn@") {
 		fd, err := strconv.Atoi(bind[8:])

bind/einhorn.go

@@ -65,6 +65,7 @@ func einhornBind(n int) (net.Listener, error) {
 
 	fno := einhornFdMap(n)
 	f := os.NewFile(uintptr(fno), fmt.Sprintf("einhorn@%d", n))
+	defer f.Close()
 	return net.FileListener(f)
 }