find_login_id() will now return 0 if login ID is an invalid user number

If login by number is supported and a client attempts login with an invalid usernumber, don't log an error, e.g.
mail 3264 SMTPS [46.148.x.x] !ERROR -2 getting data on user (6123)

But rather treat it as an invalid login attempt.

Also change lastuser() and total_users() to return int instead of uint. 2 billion users should be plenty.