pidns: Don't have unshare(CLONE_NEWPID) imply CLONE_THREAD

I goofed when I made unshare(CLONE_NEWPID) only work in a
single-threaded process.  There is no need for that requirement and in
fact I analyzied things right for setns.  The hard requirement
is for tasks that share a VM to all be in the pid namespace and
we properly prevent that in do_fork.

Just to be certain I took a look through do_wait and
forget_original_parent and there are no cases that make it any harder
for children to be in the multiple pid namespaces than it is for
children to be in the same pid namespace.  I also performed a check to
see if there were in uses of task->nsproxy_pid_ns I was not familiar
with, but it is only used when allocating a new pid for a new task,
and in checks to prevent craziness from happening.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>