This does the following things:

 - removes the ->thread_group list and uses a new PIDTYPE_TGID pid class
   to handle thread groups. This cleans up lots of code in signal.c and
   elsewhere.

 - fixes sys_execve() if a non-leader thread calls it. (2.5.38 crashed in
   this case.)

 - renames list_for_each_noprefetch to __list_for_each.

 - cleans up delayed-leader parent notification.

 - introduces link_pid() to optimize PIDTYPE_TGID installation in the
   thread-group case.

I've tested the patch with a number of threaded and non-threaded
workloads, and it works just fine. Compiles & boots on UP and SMP x86.

The session/pgrp bugs reported to lkml are probably still open, they are
the next on my todo - now that we have a clean pidhash architecture they
should be easier to fix.