replace unreachable returns with panics

Not only is this a more common idiom, it'll make finding bugs easier,
and it'll make porting to Go 1.1 easier.

Go 1.1 will not require the final return or panic because it has a
notion of terminating statements.

container.go

@@ -626,7 +626,7 @@ func (container *Container) WaitTimeout(timeout time.Duration) error {
 	case <-done:
 		return nil
 	}
-	return nil
+	panic("unreachable")
 }
 
 func (container *Container) EnsureMounted() error {

network.go

@@ -184,7 +184,7 @@ func (alloc *PortAllocator) Release(port int) error {
 	default:
 		return errors.New("Too many ports have been released")
 	}
-	return nil
+	panic("unreachable")
 }
 
 func newPortAllocator(start, end int) (*PortAllocator, error) {

utils.go

@@ -202,7 +202,7 @@ func (r *bufReader) Read(p []byte) (n int, err error) {
 		}
 		r.wait.Wait()
 	}
-	return
+	panic("unreachable")
 }
 
 func (r *bufReader) Close() error {