Wrapper for PAM

[Alexander Zhirov]

I tried to build a class with a private function 
`conversation_func` to call it inside the public authentication 
function. When compiling I get this message:
```
source/login/auth.d(87,46): Deprecation: casting from extern (C) 
int delegate(int num_msg, const(pam_message**) msg, 
pam_response** resp, void* appdata_ptr) to extern (C) int 
function(int, const(pam_message**), pam_response**, void*)* is 
deprecated
```
And when I start up, at the moment of calling the authentication 
function, I get `Segmentation fault`.

Is the function identified differently in the class? How can this 
be fixed?