groupchat: implement correct user-to-channel mapping for join responses
Each user (uib node) has a numeric ID attribute. Channels (chanib node) have
one or more user maps (aib node) with a key and value. The keys seem to be
magic numbers (normal user, channel operator?). The value is a comma separated
list of user IDs.
chatserver_response_join() now first processes the uib nodes to generate a map
and then the chanib nodes.
Based on the "guessed" meaning of the magic numbers we also set the chanop flag
for those users.