tox.*, DHT.*;
- second set of *_bootstrap_ex() functions to allow addresses instead of IPs

network.h:
- new message ID for sending/receiving IPv6 nodes

DHT.*:
- get_close_nodes():
  - additional parameter to select which kind of nodes: IPv4 or IPv6

- sendnodes()/handle_sendnodes():
  - the IPv4 message must remain intact even when the Node_format structure (which embeds IP_Port) is expanded
  - added Node4_format to keep the structure "alive"
  - copying in/out of wider to smaller format required in sendnodes()/handle_sendnodes()
  - switch of Node_format from Node4_format to Node46_format by define