send Name message when a peer rejoins

This handles the case that one peer is frozen to a second, but not vice versa.

docs/minpgc.md

@@ -35,7 +35,7 @@ and not currently mentioned in the spec.)
 
 If we receive a Rejoin packet from a peer we update its dht key, add a
 temporary groupchat connection for the peer, and, once the connection is
-online, send out a New Peer message announcing the peer.
+online, send out a New Peer message announcing the peer, and a Name message.
 
 Whenever we make a new friend connection, we check if the public key is that 
 of any frozen peer. If so, we send it a Rejoin packet, add a temporary

toxcore/group.c

@@ -1831,6 +1831,8 @@ static int handle_packet_online(Group_Chats *g_c, int friendcon_id, const uint8_
         if (peer_index != -1) {
             group_new_peer_send(g_c, groupnumber, g->group[peer_index].peer_number, real_pk, temp_pk);
         }
+
+        group_name_send(g_c, groupnumber, g_c->m->name, g_c->m->name_length);
     }
 
     ping_groupchat(g_c, groupnumber);
@@ -2379,8 +2381,9 @@ static void handle_message_packet_group(Group_Chats *g_c, uint32_t groupnumber,
     const int index = note_peer_active(g_c, groupnumber, peer_number, userdata);
 
     if (index == -1) {
-        /* We don't know the peer this packet came from so we query the list of peers from that peer.
-           (They would not have relayed it if they didn't know the peer.) */
+        /* If we don't know the peer this packet came from, then we query the
+         * list of peers from the relaying peer.
+         * (They would not have relayed it if they didn't know the peer.) */
         send_peer_query(g_c, g->close[close_index].number, g->close[close_index].group_number);
         return;
     }