Update autotopic.c

Version Alpha 0.2.0
functional changes:
+ Add join/nojoin options to "autotopic" command to determine whether or not the topic gets set every time someone joins the channel.
+ As part of that change, replace the v0.1 single chatroom preference with a preference tree for each chatroom, containing a "topic" string and "set_on_buddy_join" boolean sub-preference.
code changes:
+ Add preference macros for the "topic" and "set_on_buddy_join" sub-preferences.
+ Change autotopic_get_topic to return the "topic" sub-preference.
+ Add autotopic_get_set_on_join to return the "set_on_buddy_join" sub-preference.
+ Change autotopic_set_topic to require the topic as a passed-in argument.  Create the chatroom preference tree if needed, defaulting the set_on_buddy_join sub-preference to FALSE.
+ Add autotopic_set_set_on_join to set the "set_on_buddy_join" sub-preference.  Create the chatroom preference tree if needed, defaulting the topic sub-preference to the chatroom's current topic.
+ Change autotopic_remove_topic to remove the sub-preferences also.
+ Fix calls to autotopic_set_topic in autotopic_handle_topic_change and autotopic_cmd_cb to pass in the new topic.
+ Change AUTOTOPIC_CMD_HELP to separately list on|off, status, and join|nojoin options.
+ Change autotopic_cmd_cb to support new functionality:  "status" will report if autotopic will set the topic on new user joins; "join" and "nojoin" will turn autotopic on and set the set_on_join preference appropriately.
+ Add code to init_prefs to convert v0.1 preferences.  (Not really needed, but useful for future reference if other conversions are needed)