Refactor node ID handling, use check digits (fixes #269)

New node ID:s contain four Luhn check digits and are grouped
differently. Code uses NodeID type instead of string, so it's formatted
homogenously everywhere.

cmd/syncthing/usage_report.go

@@ -23,7 +23,7 @@ var stopUsageReportingCh = make(chan struct{})
 
 func reportData(m *model.Model) map[string]interface{} {
 	res := make(map[string]interface{})
-	res["uniqueID"] = strings.ToLower(certID([]byte(myID)))[:6]
+	res["uniqueID"] = strings.ToLower(myID.String()[:6])
 	res["version"] = Version
 	res["longVersion"] = LongVersion
 	res["platform"] = runtime.GOOS + "-" + runtime.GOARCH