xfer: Support IPv6 DCC.

There are two primary changes to weechat_network_connect_to():
* Because a ulong obviously can't transfer an IPv6 address, struct
  addrinfo* is used instead, and the port parameter is unused as it
  would be redundant, ignored, or confusing.
* Because the proxy may be at an IPv4 address and the target IPv6--or
  vice versa--the socket() call must be made in network_connect_to()
  so that it is created in the correct family. So network_connect_to()
  now returns the socket fd.

struct t_xfer is modified: local/remote_address are no longer ulong,
but instead struct sockaddr_storage. local/remote_addrlen and
local/remote_address_str fields are also added.