unloading shared objects

[Jim Blandy]

> With faster machines and auto-solib-add 0 we might get away with this
> approach (although using the old way and a smarter solib algorithm
> might have been beneficial for mozilla debugging :-).

Yeah, I'd actually say it this way: even with faster machines and
auto-solib-add 0, we're not getting away with it.  :)

At present, GDB assumes that we know the shared object's load address
when we read the symbols.  Perhaps this is a poor assumption, but it
is pretty fundamental to the current symbol table structure.  I'm not
going to fix that as part of this dlclose change.