Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
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
```
I was looking for a quick way to gauge the link quality between two nodes,
so I added the rssi to the output of the ping6 command.
Maybe this is generally useful to have.
Loosely based on [the original ping] and [netutil]'s ping
New features (compared to old RIOT version):
- non-positional parameters
- Better duplicate detection (addresses #9387)
- Better asynchronous behavior
- Potential for future move to `sock_ip`
- (Optional) DNS-support
- Multithreading-safe (in case shell-command handler gets called
from multiple threads)
[the original ping]: http://ftp.arl.army.mil/~mike/ping.html
[netutil]: https://www.gnu.org/software/inetutils/
