vma-iter, get-rusage-as: Add OpenBSD support.

vma-iter, get-rusage-as: Add OpenBSD support.

* modules/vma-iter (configure.ac): Test for mquery.
* lib/vma-iter.h (VMA_ITERATE_SUPPORTED): Define also on OpenBSD.
* lib/vma-iter.c: Include <sys/mman.h>.
(vma_iterate): Add an implementation based on mquery().
* lib/resource-ext.h (get_rusage_as): Update comments.
* lib/get-rusage-as.c: Likewise.
* lib/get-rusage-data.c: Likewise.