This is the second iteration of thread-aware coredumps.

Changes:

- Ulrich Drepper has reviewed the data structures and checked actual
  coredumps via readelf - everything looks fine and according to the spec.

- a serious bug has been fixed in the thread-state dumping code - it was
  still based on the 2.4 assumption that the task struct points to the
  kernel stack - it's task->thread_info in 2.5. This bug caused bogus
  register info to be filled in for threads.

- properly wait for all threads that share the same MM to serialize with
  the coredumping thread. This is CLONE_VM based, not tied to
  CLONE_THREAD and/or signal semantics, ie. old-style (or different-style)
  threaded apps will be properly stopped as well.

  The locking might look a bit complex, but i wanted to keep the
  __exit_mm() overhead as low as possible. It's not quite trivial to get
  these bits right, because 'sharing the MM' is detached from signals
  semantics, so we cannot rely on broadcast-kill catching all threads. So
  zap_threads() iterates through every thread and zaps those which were
  left out. (There's a minimal race left in where a newly forked child
  might escape the attention of zap_threads() - this race is fixed by the
  OOM fixes in the mmap-speedup patch.)

- fill_psinfo() is now called with the thread group leader, for the
  coredump to get 'process' state.

 - initialize the elf_thread_status structure with zeroes.

the IA64 ELF bits are not included, yet, to reduce complexity of the
patch. The patch has been tested on x86 UP and SMP.