Group chat: Add a nickname message. Remove strange default nickname. (Seriously...)

group_chats.*:
- group_send_nick() to send own name
- setnick() to store a received name

Messenger.c:
- group_send_nick() before group_sendmessage() (in regular intervals, to inform new peers)

nTox.c:
- print_groupmessage(): on error or on a name of length zero the result of tox_group_peername() isn't null-terminated, catch that