IPv6 Link-local addresses don`t work on Linux systems

[stanmcs]

Issue description

Fix in #3716 introduces new problem in ZeroMq 4.3.3.
It is not possible to connect by using IPv6 link-local address.

Part of ip_resolver.cpp

unsigned int zmq::ip_resolver_t::do_if_nametoindex (const char *ifname_)
{
#if _WIN32_WINNT > _WIN32_WINNT_WINXP && !defined ZMQ_HAVE_WINDOWS_UWP         \
  && !defined ZMQ_HAVE_VXWORKS
    return if_nametoindex (ifname_);
#else
    LIBZMQ_UNUSED (ifname_);
    // The function 'if_nametoindex' is not supported on Windows XP.
    // If we are targeting XP using a vxxx_xp toolset then fail.
    // This is brutal as this code could be run on later windows clients
    // meaning the IPv6 zone_id cannot have an interface name.
    // This could be fixed with a runtime check.
    return 0;
#endif
}

Probably bug is also present in udp_address.cpp

#if _WIN32_WINNT > _WIN32_WINNT_WINXP && !defined ZMQ_HAVE_WINDOWS_UWP         \
  && !defined ZMQ_HAVE_VXWORKS
            _bind_interface = if_nametoindex (src_name.c_str ());
            if (_bind_interface == 0) {
                //  Error, probably not an interface name.
                _bind_interface = -1;
            }
#endif

Environment

• libzmq version 4.3.3
• OS: Ubuntu 20.04

Minimal test code / Steps to reproduce the issue

Use zmq_connect with IPv6 link local address like:

zmq_connect (requester, "tcp://[fe80::213:b4ff:feff:ff02%br-9bd5d0e6c8ca]:1234")

Its not possible to connect on Linux systems.

What's the actual result? (include assertion message & call stack if applicable)

Function do_if_nametoindex always returns 0.

What's the expected result?

Function do_if_nametoindex should return network interface ID.

[bluca]

Could you send a PR to fix it?