* libltdl/slist.c, libltdl/slist.h: Merge in changes from latest

* libltdl/slist.c, libltdl/slist.h: Merge in changes from latest
upstream.  Mostly comments, formal item boxing, a sort function,
and const madness reduction.
(slist_new): Removed.
(slist_box, slist_unbox, slist_sort): New.
(SListCompare, SListCallback): Swapped!
(slist_remove, slist_find): Change order of parameters for
orthogonality with slist_foreach.  Changed all callers.
* libltdl/lt_dlloader.c (loader_cmp): Renamed to...
(loader_callback): ...this.  Return boxed item.
(lt_dlloader_remove): Adjust to new loader_callback semantics;
unbox each removed item before returning.
Remove unused variable.
Remove const from name parameter, since the slist API cannot
guarantee userdata const-ancy for its callback functions.
(lt_dlloader_find): Adjust to new loader_callback semantics; need
to return the contents of the boxed item.
Remove const from name parameter, since the slist API cannot
guarantee userdata const-ancy for its callback functions.
* libltdl/lt_dlloader.h (lt_dlloader_find, lt_dlloader_remove):
Adjust to new constless footprint.
* libltdl/ltdl.c (ltdl_exit): The global `loaders' list is changed
variable `loader' is invalidated.  Since some loaders may be
resident modules that cannot be unloaded (though we have none
yet), we must save each `next' address before calling
`lt_dlloader_remove'.
* NEWS: Updated.
* THANKS: Added Ralf.