Patch to allow DHT_bootstrap to also do LAN discovery.

This patch inserts LAN discovery in DHT_bootstrap, allowing it to find clients even if it's run parameterless (and clients don't connect to it directly, e.g. because the port is already bound to a different client).

- moves the #define of LAN_DISCOVERY_INTERVAL from Messenger.c to LAN_discovery.h
- includes LAN_discovery.h into DHT_bootstrap.c
- DHT_bootstrap.c sends and accepts LAN_discovery packets
Merge remote-tracking branch 'upstream/master' into DHT_bootstrap

Conflicts:
	toxcore/Messenger.c
Merge pull request #575 from FullName/DHT_bootstrap

Patch to allow DHT_bootstrap to also do LAN discovery.
Very basic group chats, tested and working with nTox.

Please wait until the tox.h API is updated before integrating it into
your clients.

nTox:
/g
creates a new group chat

/i friendnum groupnum
invite friendnum to groupnum

/z groupnum message
send message to groupnum

NOTE: group chats currenly might not handle packet loss well if there are less than 6 participants.
Merge branch 'master' of https://github.com/irungentoo/ProjectTox-Core
Put group chat functions in the public API.

Group chats are not complete, they seem to work very well though.

This means that the functions will change.
TOX_ENABLE_IPV6 is now #define'd per default. Added some logging and error messages.
Merge remote-tracking branch 'upstream/master' into Integration