unwind support for Linux 2.6 vsyscall DSO

[Roland McGrath]

> It is useful if you want to apply a specific function to the entries
> of the auxv vectors in a generic way while iterating. There are plenty
> of other examples in gdb and bfd where this interface is used, albeit
> it may not be the most efficient. I think you could still write an
> iterator because it will be needed.

I understand.  I would prefer a block-reading interface in the target
vector (e.g. something similar to target_read_aux_vector as in the patch I
posted) and a utility function that is an iterator applied to the block.
Do you see a problem with that?  Or do you think that the conversion from
target width and byte order to host format and back is desireable in some
way?  I don't feel strongly about this.