Avoid mixing functions and data pointers in callback functions.

Avoid mixing functions and data pointers in callback functions.
* libltdl/ltdl.c (file_worker_func): New type.
(lt_dlforeachfile): Instead of passing a function pointer as a
data pointer, pass a pointer to a file_worker_func pointer.
(foreach_callback_func): Adjust.
Backport from 2006-08-31 fix in HEAD.
Report by Maciej Bliziński <maciej.blizinski@gmail.com>.