If there is more than one interface, print the id of the receiving
interface for convenience.
```
ping6 ff02::1
2020-02-20 18:19:38,644 # 12 bytes from fe80::d0ae:c1b:2054:58e%7: icmp_seq=0 ttl=64 rssi=-45 dBm time=4.801 ms
2020-02-20 18:19:38,652 # 12 bytes from fe80::d0ae:c1b:2054:58c%8: icmp_seq=0 ttl=64 rssi=-63 dBm time=13.422 ms (DUP!)
2020-02-20 18:19:38,661 # 12 bytes from fe80::c78:16d9:8aca:ba9a%8: icmp_seq=0 ttl=64 rssi=-44 dBm time=21.819 ms (DUP!)
2020-02-20 18:19:39,648 # 12 bytes from fe80::d0ae:c1b:2054:58e%7: icmp_seq=1 ttl=64 rssi=-44 dBm time=4.797 ms
2020-02-20 18:19:39,657 # 12 bytes from fe80::d0ae:c1b:2054:58c%8: icmp_seq=1 ttl=64 rssi=-61 dBm time=12.917 ms (DUP!)
2020-02-20 18:19:39,666 # 12 bytes from fe80::c78:16d9:8aca:ba9a%8: icmp_seq=1 ttl=64 rssi=-43 dBm time=21.679 ms (DUP!)
2020-02-20 18:19:40,657 # 12 bytes from fe80::d0ae:c1b:2054:58e%7: icmp_seq=2 ttl=64 rssi=-41 dBm time=4.795 ms
```
Having the definitions sit in the `net/gnrc/sixlowpan/frag.h` header
does not make much sense, when using Selective Fragment Forwarding
(and the fragmentation buffer already includes a
`net/gnrc/sixlowpan/frag/stats.h` header), so they are moved to their
own header. Since with this change it makes more sense to have the
statistics stored in their own sub-module, the pseudo-module is also
actualized.
Previously `ifconfig` would only know link-local addresses
(printed as 'local') and everything else would be 'global'.
This is wrong for site-local and unique local addresses which were
also denoted as global.
So use the already existing helper functions to determine the correct
type of IPv6 address when printing.
This change is in preparation to [PR 10788]. PR 10788 will make the
shell exitable which may lead to unexpected behavior in comparison to
previous usage of the shell.
To prevent this, this PR introduces two "new" functions to the shell's
API: `shell_run_once()` and `shell_run_forever()`.
`shell_run_once()` basically has the same behavior as `shell_run()` in
current master: Start a shell and continue reading lines until EOF is
reached.
`shell_run_forever()` wraps around `shell_run_once()` and restarts the
shell if it exits.
`shell_run()` is re-introduced as a back-porting alias for
`shell_run_forever()`.
As a consequence all current calls to `shell_run()` won't exit even
with [PR 10788] merged (which would add EOT as additional exit
condition for `shell_run_once()`).
[PR 10788]: https://github.com/RIOT-OS/RIOT/pull/10788
