reloadconfig fails if circus is managing circushttpd daemon

When we have httpd = True on [circus] section, reloadconfig fails with the error below. 

Environ:
* circus==0.9.2 (installed via pip  / pypi)
* python 2.7
* Reproduced on ubuntu 12.04 and RHEL5


Circus debug log: 

```
2013-09-24 16:42:42 [5369] [DEBUG] got message {"command": "reloadconfig", "properties": {}}
2013-09-24 16:42:42 [5369] [DEBUG] Reading config files: ['/etc/circus/cams.d/jamaica-1.ini']
2013-09-24 16:42:42 [5369] [DEBUG] error: command '{"command": "reloadconfig", "properties": {}}': Watchers set(['circushttpd']) uses a socket which is deleted

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/circus/controller.py", line 125, in dispatch
    resp = cmd.execute(self.arbiter, properties)
  File "/usr/local/lib/python2.7/dist-packages/circus/commands/reloadconfig.py", line 41, in execute
    arbiter.reload_from_config()
  File "/usr/local/lib/python2.7/dist-packages/circus/arbiter.py", line 314, in reload_from_config
    wn_with_deleted_socket)
ValueError: Watchers set(['circushttpd']) uses a socket which is deleted
```

* circus.ini:

```
[circus]
statsd = True
httpd = True
httpd_host = 0.0.0.0
httpd_port = 8080
include = /etc/circus/cams.d/*.ini
logoutput = /var/log/circusd-output.log
loglevel = DEBUG

```