version skipping

[Roland McGrath]

> I've checked in the files Roland had in his initial mail.  The code
> contained a few bugs bug after fixing them it seems to work now.  At
> least for the case where no code is elided.

I've done some tests in cases where code is elided, including at least a
nominal test of skipped-in-the-middle.

> One more thing I want to see implemented: Let people overwrite the
> decision through configure.  At least let it be more restrictive.  The
> reason for that is that in very limited environments where all the
> applications running are under control one might be sure to not have
> to care about old code.

I'd suggest doing this by just specifying a file (or list of files)
to prepend to shlib-versions in the processing.