Set link state correctly in lwIP for interfaces that support
the NETOPT_LINK option. Interfaces that do not support it
(like tap for native arch) remain up all the time.
Netdevs that support NETOPT_LINK but do not send NETDEV_EVENT_LINK_UP/DOWN
events will end up with a mismatched link state - but DHCP would
already not start for them either.
Use netifapi to signal lwIP to do the work in its own thread.
Lists state, link type, v4/v6 addresses.
Currently read-only.
Using lwIP debug system to print addresses, to limit dependencies
and work with dual stack setup. Most other code seems to only
allow either v4 or v6 networking. For that to compile I
had to change the `SZT_F` format string due to this error:
```
error: format '%lu' expects argument of type 'long unsigned int',
but argument 2 has type 'size_t {aka unsigned int}'
```
Switching to the lwIP default format string here.
Outputs the following on my ESP32 board with Ethernet,
when both v4 and v6 are enabled in examples/paho-mqtt:
```
> ifconfig
Iface ET0 HWaddr: 24:0a:c4:e6:0e:9f Link: up State: up
Link type: wired
inet addr: 10.4.4.81 mask: 255.255.254.0 gw: 10.4.4.1
inet6 addr: fe80:0:0:0:260a:c4ff:fee6:e9f scope: link
inet6 addr: 2001:db8:1000:0:260a:c4ff:fee6:e9f scope: global
Iface ET1 HWaddr: 24:0a:c4:e6:0e:9c Link: up State: up
Link type: wireless
inet addr: 10.4.4.82 mask: 255.255.254.0 gw: 10.4.4.1
inet6 addr: fe80:0:0:0:260a:c4ff:fee6:e9c scope: link
inet6 addr: 2001:db8:1000:0:260a:c4ff:fee6:e9c scope: global
>
```
Either the sock is provided with `sock_*_send()` or not. In the first
case the indirection is not necessary, and in the second we need to
delete the created `conn` within `lwip_sock_send()` anyway, so returning
it makes no sense.
To avoid priority conflicts with the WiFi hardware driver thread which has priority of 1, the default thread priority of lwIP's TCP/IP thread is decreased to 2.
If module log is used, log.h should be included. Otherwise, it might lead to compilation problems since LOG_ERROR macro isn't know in expansion of LWIP_PLATFORM_ASSERT macro.
