A revised patch for dlclose

[Jim Blandy]

> > If each DSO was given its own obstack, it would be pretty easy.
> 
> This is largely a solved problem (via objfiles).
> [...]
> There are a few loose ends in freeing, but it is the tangled logic in
> find_solib that is tripping us up more than the freeing.

Yes, that's right.

I've pretty much come to the same conclusion as JimK --- I think there
are ways to repartition find_solib's responsibilities that will make
this logic a lot simpler.