They make a full (deep) copy of a list.

In contrast with g_[s]list_copy(), these functions take a function as a argument
to make a copy of each list element, in addition to copying the list container itself.

The functions g_[s]list_copy() were reimplemented to just call the new functions
with NULL as the function argument, which will behave like current implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=675024