common/httpserver: listen on an abstract Unix socket

And make healthcheck command use it by default. This makes the
healthcheck command works whatever port the user has configured for the
HTTP service.

common/httpserver/cache_test.go

@@ -139,7 +139,7 @@ func TestRedis(t *testing.T) {
 		Server:   server,
 		DB:       10,
 	}
-	h, err := httpserver.New(r, config, httpserver.Dependencies{Daemon: daemon.NewMock(t)})
+	h, err := httpserver.New(r, "cache-test", config, httpserver.Dependencies{Daemon: daemon.NewMock(t)})
 	if err != nil {
 		t.Fatalf("New() error:\n%+v", err)
 	}