shell/gnrc_icmpv6_echo: test for ICMPv6 echo reply corruption
shell/gnrc_icmpv6_echo: test for ICMPv6 echo reply corruption

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.

jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT