Group chat: load/save

Messenger.c:
- size&store groupchats, load back with load_groupchat()
- fix in #LOGGING: correctly convert an ID to a string

group_chats.*:
- load_groupchat(): converts the stored bytes back into a chat structure
- handle_data::switch(packet type): don't fall-through from case 0 (ping) to case 16 (new peer) (yet peer case exits from fall-through at once)

tox.*:
- offer the number of group chats (ignores invalid chats, so only useful after loading)

DHT.c:
- enclose a fprintf(stderr, ...) in #DEBUG