Test and fix slist.c.

Test and fix slist.c.

* libltdl/libltdl/slist.h: Include stddef.h, for size_t.
(slist_remove): Return pointer to SList, not void.
* libltdl/slist.c: Include stdlib.h, for malloc and free.
(slist_remove): Adjust prototype as above.
(slist_sort): Do not loop forever on one-item list.
* tests/slist.at (SList functionality): New file, new test.
* Makefile.am (TESTSUITE_AT): Add tests/slist.at.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>