elf: Fix force_first handling in dlclose (bug 30981)

The force_first parameter was ineffective because the dlclose'd
object was not necessarily the first in the maps array.  Also
enable force_first handling unconditionally, regardless of namespace.
The initial object in a namespace should be destructed first, too.

The _dl_sort_maps_dfs function had early returns for relocation
dependency processing which broke force_first handling, too, and
this is fixed in this change as well.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>