WIP Rewrite uv_udp_bind.

uv_udp_bind(handle, port)  binds now in dual stack mode,
uv_udp_bind4(handle, addr) only to ipv4 addresses and
uv_udp_bind6(handle, addr6) only to ipv6 addresses.

We don't need the flags attribute anymore, but have instead have
an additional method.

What do you think about the API?