Add up resource usage counters for live and dead threads to show aggregate
per-process usage in /proc/<pid>/stat.  This mirrors the new getrusage()
semantics.  /proc/<pid>/task/<tid>/stat still has the per-thread usage.

After moving the counter aggregation loop inside a task->sighand lock to
avoid nasty race conditions, it has survived stress-testing with '(while
true; do sleep 1 & done) & top -d 0.1'
Signed-off-by: Lev Makhlis <mlev@despammed.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>