Advertise feature in disco info for account JID

As per XEP-0313 >= 0.2, advertise the MAM feature in the service
discovery information for the bare account JID.

Some clients check the server's discovery information instead, so we'll
continue to advertise the feature there as well (at least for the
moment).

mod_mam/src/mod_mam.erl

@@ -50,7 +50,8 @@
 	 code_change/3]).
 
 %% ejabberd_hooks callbacks.
--export([receive_stanza/4,
+-export([disco_features/5,
+	 receive_stanza/4,
 	 send_stanza/3,
 	 remove_user/2]).
 
@@ -157,7 +158,9 @@ start(Host, Opts) ->
 			     parallel),
     gen_iq_handler:add_iq_handler(ejabberd_sm, Host, ?NS_MAM, ?MODULE,
 				  handle_iq, IQDisc),
+    %% Set up MAM feature announcement.
     mod_disco:register_feature(Host, ?NS_MAM),
+    ejabberd_hooks:add(disco_sm_features, Host, ?MODULE, disco_features, 50),
     %% Set up message storage process.
     Proc = gen_mod:get_module_proc(Host, ?PROCNAME),
     Spec = {Proc, {?MODULE, start_link, [Host, Opts]}, permanent, 3000, worker,
@@ -167,6 +170,8 @@ start(Host, Opts) ->
 -spec stop(binary()) -> ok.
 
 stop(Host) ->
+    ejabberd_hooks:delete(disco_sm_features, Host, ?MODULE, disco_features, 50),
+    %% Stop message storage process.
     Proc = gen_mod:get_module_proc(Host, ?PROCNAME),
     ok = supervisor:terminate_child(ejabberd_sup, Proc),
     ok = supervisor:delete_child(ejabberd_sup, Proc).
@@ -258,6 +263,17 @@ code_change(_OldVsn, #state{host = Host} = State, _Extra) ->
 %% ejabberd_hooks callbacks.
 %%--------------------------------------------------------------------
 
+-spec disco_features(empty | {result, [binary()]}, jid(), jid(), binary(),
+		     binary()) -> {result, [binary()]}.
+
+disco_features(empty, From, To, Node, Lang) ->
+    disco_features({result, []}, From, To, Node, Lang);
+disco_features({result, OtherFeatures},
+	       #jid{luser = U, lserver = S},
+	       #jid{luser = U, lserver = S}, <<"">>, _Lang) ->
+    {result, OtherFeatures ++ [?NS_MAM]};
+disco_features(Acc, _From, _To, _Node, _Lang) -> Acc.
+
 -spec receive_stanza(jid(), jid(), jid(), xmlel()) -> ok.
 
 receive_stanza(#jid{luser = U, lserver = S} = JID, From, To,