The copyf function should operate on `void*`

If we get/return `void const*` here, we cannot use this function to copy
values on insertion, for example.
Fix: ll_insert() to use the cfg->copyf() before adding the data element
Merge pull request #278 from matthiasbeyer/add-config_functionality_for_basic_operations

Add config functionality for basic operations
Fix: Logical OR was wrong

If `pred` is `NULL`, the line still tries to execute the function. This
is fixed by this commit.
Merge pull request #279 from matthiasbeyer/fix-ll_select

Fix: Logical OR in ll_select()
We want to call the processing function if pred is NULL

This fixes the fix introduced my Matthias Beyer.
Which I was dump enough to merge.
Merge pull request #280 from neithernut/fix-ll_select

We want to call the processing function if pred is NULL