New module 'vma-iter'.

* lib/vma-iter.h: New file.
* lib/vma-iter.c: New file, based on lib/get-rusage-as.c.
* modules/vma-iter: New file.
* lib/get-rusage-as.c: Include vma-iter.h. Don't include system headers
for get_rusage_as_via_iterator.
(vma_iterate_callback): New function.
(get_rusage_as_via_iterator): Rewritten to use vma_iterate.
* modules/get-rusage-as (Depends-on): Add vma-iter.