Fix hangs on SIGTERM.

This fixes #1469, where three bugs pervent main from existing on
SIGTERM.

The first two are blocking receiving from cancel chanels which never
have values sent to them, cancelStateUpdateChan and
cancelPolicyUpdateChan. It seems closing the chanels should be all the
is needed to signal the watching goroutines to exit.

The other is the signal hander never exiting an infinite for loop. The
sigFunc is called in an errorGroup which blocks exiting Serve and thus
main. It looks like a refactor in #1382 removed an os.Exit(0),
replacing it with a return breaks out of the loop.

hscontrol/app.go

@@ -760,7 +760,6 @@ func (h *Headscale) Serve() error {
 				// Stop listening (and unlink the socket if unix type):
 				socketListener.Close()
 
-				<-h.cancelStateUpdateChan
 				close(h.stateUpdateChan)
 				close(h.cancelStateUpdateChan)
 
@@ -775,6 +774,7 @@ func (h *Headscale) Serve() error {
 
 				// And we're done:
 				cancel()
+				return
 			}
 		}
 	}