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
Merge branch 'group_peername' of https://github.com/FullName/ProjectTox-Core into FullName-group_peername
Cleaned up the code.
Add a "conversation mode", where you set your conversation partner once and then all non-commands are sent as message to them.
/cf # rsp. /cg # sets, /cr resets target

Also reformatted display of sent group message slightly to look less beta-ish.
Then using "#<num>" for group number vs. "<num>" for friend/peer numbers.
Also changed to a slightly different message on people without name.
IPv6 address of ::1 is local.
Merge pull request #647 from FullName/ntox-conversations

Add a "conversation mode", where you set your conversation partner once ...
Merge pull request #648 from FullName/LAN_ip-ip6-loopback-is-local

IPv6 address of ::1 is local.
Merge remote-tracking branch 'irungentoo/master'