cmd/syncthing: Extract interfaces for things the API depends on

Enables testing of the API service, in the long run.

cmd/syncthing/usage_report.go

@@ -79,7 +79,7 @@ func (m *usageReportingManager) String() string {
 
 // reportData returns the data to be sent in a usage report. It's used in
 // various places, so not part of the usageReportingManager object.
-func reportData(cfg *config.Wrapper, m *model.Model) map[string]interface{} {
+func reportData(cfg configIntf, m modelIntf) map[string]interface{} {
 	res := make(map[string]interface{})
 	res["urVersion"] = usageReportVersion
 	res["uniqueID"] = cfg.Options().URUniqueID