Fix for an issue where re-started sub-processes would inherit

signal handlers from the process manager. Then you would end up with stack traces like:

```
Process Publisher-6:
Traceback (most recent call last):
  File "/usr/lib64/python2.6/multiprocessing/process.py", line 232, in _bootstrap
    self.run()
  File "/home/thjackso/src/salt-github/salt/master.py", line 403, in run
    package = pull_sock.recv()
  File "socket.pyx", line 628, in zmq.backend.cython.socket.Socket.recv (zmq/backend/cython/socket.c:5616)
  File "socket.pyx", line 662, in zmq.backend.cython.socket.Socket.recv (zmq/backend/cython/socket.c:5436)
  File "socket.pyx", line 139, in zmq.backend.cython.socket._recv_copy (zmq/backend/cython/socket.c:1771)
  File "checkrc.pxd", line 11, in zmq.backend.cython.checkrc._check_rc (zmq/backend/cython/socket.c:5863)
  File "/home/thjackso/src/salt-github/salt/utils/process.py", line 242, in kill_children
    p_map['Process'].join(0)
  File "/usr/lib64/python2.6/multiprocessing/process.py", line 117, in join
    assert self._parent_pid == os.getpid(), 'can only join a child process'
AssertionError: can only join a child process
```