addto_lists(): store the IP/Port that was used to *send*.

Avoids a DOS of sending a copy of a valid response with an invalid IP.
Unify most of handle_sendnodes[_ipv6]() into a common handle_sendnodes_core(), leave only a minimal trail.
Merge pull request #642 from FullName/dht_handle_sendnodes_unify

Unify most of handle_sendnodes[_ipv6]() into a common core.
Merge branch 'add2lists_forward' of https://github.com/FullName/ProjectTox-Core into FullName-add2lists_forward
assoc.h, Messenger.c, DHT.c:
- tribute to rendezvous
- self_client_id is now taken from dht instead of from Assoc_self()
- Assoc_self_client_id_changed() to notify when the client_id changed

assoc.c:
- dhtassoc->assoc (+ some assoc->ipptp)
- self_client_id now pointer
- self_hash marks if self_client_id is valid
Merge remote-tracking branch 'upstream/master' into ID-IP-r1

Conflicts:
	toxcore/DHT.c
	toxcore/ping.c
Code cleanups.
Merge remote-tracking branch 'upstream/master' into ID-IP-r1
Fix off-by-one error.