progs/pixie.c: Rewrite list hacking to avoid strict-aliasing badness.

The circular list stuff was quite pretty but involved some really
unpleasant casting which modern GCC (quite properly) complains about
vociferously.

Replace it with more traditional doubly-linked-list hacking with
null-pointer sentinels, with the slightly nasty pointer swizzling tucked
away in useful macros.  Some of the uses of these macros (e.g.,
unlinking the first or last item in a list) could be made more efficient
by using special-case versions, but it doesn't seem worthwhile.