Fix probe & app's shutdown to please signals.SignalHandlerLoop

Fixes:
```
not enough arguments in call to signals.SignalHandlerLoop
	have ()
	want (logging.Interface, ...signals.SignalReceiver)
```

probe/probe.go

@@ -110,9 +110,10 @@ func (p *Probe) Start() {
 }
 
 // Stop stops the probe
-func (p *Probe) Stop() {
+func (p *Probe) Stop() error {
 	close(p.quit)
 	p.done.Wait()
+	return nil
 }
 
 // Publish will queue a report for immediate publication,

prog/app.go

@@ -19,6 +19,7 @@ import (
 
 	billing "github.com/weaveworks/billing-client"
 	"github.com/weaveworks/common/aws"
+	"github.com/weaveworks/common/logging"
 	"github.com/weaveworks/common/middleware"
 	"github.com/weaveworks/common/network"
 	"github.com/weaveworks/common/signals"
@@ -307,10 +308,27 @@ func appMain(flags appFlags) {
 	}()
 
 	// block until INT/TERM
-	signals.SignalHandlerLoop()
+	signals.SignalHandlerLoop(
+		logging.Logrus(log.StandardLogger()),
+		stopper{
+			Server:      server,
+			StopTimeout: flags.stopTimeout,
+		},
+	)
+}
+
+// stopper adapts graceful.Server's interface to signals.SignalReceiver's interface.
+type stopper struct {
+	Server      *graceful.Server
+	StopTimeout time.Duration
+}
+
+// Stop implements signals.SignalReceiver's Stop method.
+func (c stopper) Stop() error {
 	// stop listening, wait for any active connections to finish
-	server.Stop(flags.stopTimeout)
-	<-server.StopChan()
+	c.Server.Stop(c.StopTimeout)
+	<-c.Server.StopChan()
+	return nil
 }
 
 func newWeavePublisher(dockerEndpoint, weaveAddr, weaveHostname, containerName string) (*app.WeavePublisher, error) {

prog/probe.go

@@ -14,6 +14,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	log "github.com/sirupsen/logrus"
 
+	"github.com/weaveworks/common/logging"
 	"github.com/weaveworks/common/network"
 	"github.com/weaveworks/common/sanitize"
 	"github.com/weaveworks/common/signals"
@@ -304,7 +305,8 @@ func probeMain(flags probeFlags, targets []appclient.Target) {
 	maybeExportProfileData(flags)
 
 	p.Start()
-	defer p.Stop()
-
-	signals.SignalHandlerLoop()
+	signals.SignalHandlerLoop(
+		logging.Logrus(log.StandardLogger()),
+		p,
+	)
 }