shell_commands: adapt ping6 to original-ping-like implementation

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/