Disable use of signals in Curl

Various places suggest that this is needed for better thread-safety,
and the only drawback is that some systems cannot timeout properly on
DNS lookups.

nilmdb/client/httpclient.py

@@ -23,6 +23,7 @@ class HTTPClient(object):
         self.curl.setopt(pycurl.SSL_VERIFYHOST, 2)
         self.curl.setopt(pycurl.FOLLOWLOCATION, 1)
         self.curl.setopt(pycurl.MAXREDIRS, 5)
+        self.curl.setopt(pycurl.NOSIGNAL, 1)
         self._setup_url()
 
     def _setup_url(self, url = "", params = ""):