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.
Nonce generation changes.

Nonces don't need to be random, only different.

also random_int now gives same quality random numbers for both NaCl and
libsodium.