kernel - Fix signal races against exiting processes

kernel - Fix signal races against exiting processes

* Fix a panic which can occur when a signal races an exiting process.

* Fix an incorrect status return from the wait*() system calls which
  can sometimes occur when a SIGTSTP or other signal races an exit.

  Do not allow the signal status to overwrite the exit value.

* Fix a race where the kill() system call can return ESRCH when a signal
  races an exiting process which has already entered the zombie state.

  Return success in this case and silently drop the signal.