kernel - Fix panic during coredump

kernel - Fix panic during coredump

* multi-threaded coredumps were not stopping all other threads before
  attempting to scan the vm_map, resulting in numerous possible panics.

* Add a new process state, SCORE, indicating that a core dump is in progress
  and adjust proc_stop() and friends as well as any code which tests the
  SSTOP state.  SCORE overrides SSTOP.

* The coredump code actively waits for all running threads to stop before
  proceeding.

* Prevent a deadlock between a SIGKILL and core dump in progress by
  temporarily counting the master exit thread as a stopped thread (which
  allows the coredump to proceed and finish).

Reported-by: marino