core: mark every side O_CLOEXEC

Not sure why this wasn't done before, seems it should have always been
this way, and can't see any reason it wasn't. Without it, many fds are
leaked into at least .local() children. Closes #163.

mitogen/core.py

@@ -666,6 +666,7 @@ class Side(object):
         self.stream = stream
         self.fd = fd
         self.keep_alive = keep_alive
+        set_cloexec(fd)
         set_nonblock(fd)
 
     def __repr__(self):