Mainloop: init sigchld_handler as None

There is no point in initializing sigchld_handler as an empty dict, when
it will be overwritten by an instance of utils.SignalHandler
immediately after.

Reviewed-by: iustinp

lib/daemon.py

@@ -36,7 +36,7 @@ class Mainloop(object):
   def __init__(self):
     self._io_wait = []
     self._signal_wait = []
-    self.sigchld_handler = {}
+    self.sigchld_handler = None
 
   def Run(self):
     # TODO: Does not yet support adding new event sources while running