elf: Implement force_first handling in _dl_sort_maps_dfs (bug 28937)

The implementation in _dl_close_worker requires that the first
element of l_initfini is always this very map (“We are always the
zeroth entry, and since we don't include ourselves in the
dependency analysis start at 1.”).  Rather than fixing that
assumption, this commit adds an implementation of the force_first
argument to the new dependency sorting algorithm.  This also means
that the directly dlopen'ed shared object is always initialized last,
which is the least surprising behavior in the presence of cycles.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit 1df71d32)