Main: Eliminate TOX_ENABLE_IPV6 (then always on), CLIENT_ONETOONE_IP (th...

...en always off).

Additionally (besides cleanups):

network.h/tox.h:
- WIN32: fix a strange sa_family_t definition
- WIN32: define EWOULDBLOCK to WSAEWOULDBLOCK
- WIN32: kill macro for an existing function (IN6_ADDR_EQUAL)

network.c:
- use EWOULDBLOCK instead of EAGAIN (same value, but EWOULDBLOCK is more "popular")
- new_networking(): only try to enable IPv4-in-IPv6 if it's not already enabled per default
- inet_ntop()/inet_pton(): WIN32: remove partial initializers in favor of a simple memset()
- ip_equal(): WIN32: use an existing function
- logging: networking_wait_execute(): only dump result if not timeout
- logging: loglogdata(): kill an unused variable

LAN_discovery.c:
- send_broadcasts(): re-enabled, can only support IPv4 by principle, split into fetch_broadcast_info() (to fetch the addresses once) and send_broadcasts() (actual sending)

DHT.c:
- DHT_load_state_callback(): enclosed a fprintf(stderr, ...) into #ifdef DEBUG

Lossless_UDP.c:
- change_handshake(): harden against strange sa_family_t definitions

Messenger.c:
- logging: fix ID to string conversion

util.c:
- logging: eliminate a signed-warning