qotd: Use maxlen for recv

For QOTD, 512 is not mandatory, so allow to fetch more data
if user asks (by using "-c" argument).

plugins/qotd.py

     def _fetch_qotd(self):
         s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         s.connect((self.host, self.port))
-        self.qotd = s.recv(512).strip()
+        self.qotd = s.recv(self.data.maxlen).strip()
         s.close()
 
     def render_plain(self):