dlna: add connection manager service description

The UPnP MediaServer spec says that the ConnectionManager service is
required, and adding it was enough to get dlna support working on my
other TV (LG webOS 2.2.1).

cmd/serve/dlna/dlna_test.go

@@ -59,6 +59,9 @@ func TestRootSCPD(t *testing.T) {
 	// Make sure that the SCPD contains a CDS service.
 	require.Contains(t, string(body),
 		"<serviceType>urn:schemas-upnp-org:service:ContentDirectory:1</serviceType>")
+	// Make sure that the SCPD contains a CM service.
+	require.Contains(t, string(body),
+		"<serviceType>urn:schemas-upnp-org:service:ConnectionManager:1</serviceType>")
 	// Ensure that the SCPD url is configured.
 	require.Regexp(t, "<SCPDURL>/.*</SCPDURL>", string(body))
 }