threading fixes v4

[David Bremner]

David Bremner <david at tethera.net> writes:

> This is mainly coding style issues pointed out by Tomi, along with
> moving some of the low level list access into two new functions.  The
> interdiff is a bit noisy since I dropped the creation of
> debug_print.{c,h}.
>
> I'm going to optimistically claim this version is ready to go, but
> feel free to point out any issues.

I have pushed this version. Let the complaining from non-reviewers
commence ;).

d