[RFA] inside_entry_func() related changes

[Andrew Cagney]

> 
> Revisiting this patch too.  I've added documentation and removed the
> now-unneeded change to inside_entry_func; does anyone disagree with
> this patch?  Docs OK?
> 
> A testcase would indeed be nice.  Damned if I know how to write one,
> though.

It's possible to test backtrace past main, and that the command options 
at least work.

Andrew