pid: improved namespaced iteration over processes list (v2)

[Gowrishankar M]

Earlier version of this patch, attempted to change general macros 
for_each_process and do_each_thread which is not safe as suggested
by most of reviewers.

Hence, I go with adding renamed versions of them, so that we can use
them only whereever needed.

I will include code optimisation that Dave suggested if this patch 
gets approved mostly.

Thanks.
Gowrishankar