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.