Fix reparenting to the same thread group. (take 2)

Fix reparenting to the same thread group. (take 2)

This patch fixes the case when we reparent to a different thread in the
same thread group.  This modifies the code so that we do not send
signals and do not change the signal to send to SIGCHLD unless we have
change the thread group of our parents.  It also suppresses sending
pdeath_sig in this cas as well since the result of geppid doesn't
change.

Thanks to Oleg for spotting my bug of only fixing this for non-ptraced
tasks.

This fixes the issues identified by Albert Cahalan in thread
http://lkml.org/lkml/2006/12/21/22

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>