`msg_avail()` will return -1 if the thread has no message queue.
Casting this to unsigned will result in the `ping` command to try
receiving 4294967295 messages, which hangs the shell.
Drop the cast to `unsigned` and the loop behaves as intended.
But then it's still wrong: If new messages become available, they
would be ignored.
So change the `for` loop to a `while` loop. The index variable is
not used at all.
Currently a valid netif name must be passed to show the usage
instructions:
```
> ifconfig help
error: invalid interface given
> ifconfig 6 help
usage: ifconfig
usage: ifconfig <if_id> [up|down]
[...]
```
`ifconfig --help` is also accepted.
The Linux ping utility has the nice feature that fills the ICMPv6 echo
request payload with a pattern `payload_index & 0xFF`.
Then the ICMPv6 echo response payload is checked to verify that the pattern
is still intact.
This way corrupted messages can be detected.
In the past that revealed some 6lo-fragmentation bugs in Linux when
corrupted replies arrived.
This feature is also useful for RIOT, so implement it in RIOTs `ping`
command.
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
>
```
Previously a value of 0 was used for the RSSI to signal that this value is not
present in `gnrc_netif_hdr_t`. However, an RSSI of 0 dBm is legal and even very
plausible data.
This commit defines `GNRC_NETIF_HDR_NO_RSSI` as `INT16_MIN`, which is below the
noise floor in the vacuum of outer space and hence impossible to receive.
For consistency, also GNRC_NETIF_HDR_NO_LQI is defined.
