framework: Don not catch POSIX signals
    
    Let it kill your process in the normal way
    
    See https://bugzilla.redhat.com/show_bug.cgi?id=1366004#c35
    
    Fixes:
    Traceback (most recent call last):
      File "/usr/lib64/python3.6/site-packages/dbus/service.py", line 647, in _message_cb
        if not isinstance(message, MethodCallMessage):
    SystemError: <built-in function isinstance> returned a result with an error set
    Traceback (most recent call last):
      File "/usr/lib64/python3.6/site-packages/dbus/connection.py", line 521, in _signal_func
        def _signal_func(self, message):
      File "/usr/lib/python3.6/site-packages/setroubleshoot/server.py", line 114, in sighandler
        sys.exit()