Add 'sp_loginclass' to retrieve the user's login class

This was developed and tested on OpenBSD, though I've not yet been able to verify this on other systems.

```ruby
irb(main):001:0> require 'shadow'
=> true
irb(main):002:0> Shadow::Passwd.getspnam('jasper').sp_loginclass
=> "staff"
irb(main):003:0> 
```