(PUP-3723) Add new loginclass parameter to user type

Will this work if `sp_loginclass` isn't implemented on `Shadow::Passwd` (i.e. user has version before 2.4.1)?  I don't see a check to see if the method exists on the entry where we define the method.

Also, are you aware that your commit to ruby-shadow was partially reverted in 2.4.1?  Just wanted to make sure that your changes to ruby-shadow still work as expected.