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/
