Log how many things were loaded for each domain

This patch makes chasquid log how many users, aliases and DKIM keys were
loaded for each domain.

This makes it easier to confirm changes, and troubleshoot problems
related to these per-domain configuration files.

cmd/chasquid-util/chasquid-util.go

@@ -156,12 +156,12 @@ func checkUserDB() {
 		Fatalf("Error: file %q does not exist", path)
 	}
 
-	_, err := userdb.Load(path)
+	udb, err := userdb.Load(path)
 	if err != nil {
 		Fatalf("Error loading database: %v", err)
 	}
 
-	fmt.Println("Database loaded")
+	fmt.Printf("Database loaded (%d users)\n", udb.Len())
 }
 
 // chasquid-util user-add <user@domain> [--password=<password>] [--receive_only]