Misc helper patches for pid namespaces

Some helper patches to support multiple pid namespaces. These
were posted earlier on Containers@ mailing list. 

	[PATCH 1/5] Define and use task_active_pid_ns() wrapper
	[PATCH 2/5] Rename child_reaper() function.
	[PATCH 3/5] Use task_pid() to find leader's pid
	[PATCH 4/5] Define is_global_init() and is_container_init().
	[PATCH 5/5] Move alloc_pid() to copy_process()

Changelog:

	- Addressed Oleg Nesterov's comments on [V1] of this patchset.