shell_commands/ping6: mix-in current time into ID
shell_commands/ping6: mix-in current time into ID

This prevents two fast executions of `ping6` behind each other to
confuse their respective responses.

x3ro/RIOT,toonst/RIOT,basilfx/RIOT,mtausig/RIOT,basilfx/RIOT,yogo1212/RIOT,jasonatran/RIOT,rfuentess/RIOT,smlng/RIOT,authmillenon/RIOT,miri64/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,rfuentess/RIOT,cladmi/RIOT,basilfx/RIOT,OlegHahm/RIOT,rfuentess/RIOT,toonst/RIOT,mtausig/RIOT,OTAkeys/RIOT,basilfx/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,aeneby/RIOT,OlegHahm/RIOT,kaspar030/RIOT,josephnoir/RIOT,authmillenon/RIOT,smlng/RIOT,toonst/RIOT,smlng/RIOT,toonst/RIOT,ant9000/RIOT,aeneby/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,rfuentess/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,smlng/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kaspar030/RIOT,smlng/RIOT,OTAkeys/RIOT,mtausig/RIOT,RIOT-OS/RIOT,x3ro/RIOT,jasonatran/RIOT,cladmi/RIOT,aeneby/RIOT,josephnoir/RIOT,aeneby/RIOT,yogo1212/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,x3ro/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,josephnoir/RIOT,yogo1212/RIOT,cladmi/RIOT,cladmi/RIOT,authmillenon/RIOT,aeneby/RIOT,basilfx/RIOT,x3ro/RIOT,kYc0o/RIOT,toonst/RIOT,x3ro/RIOT,yogo1212/RIOT,cladmi/RIOT,rfuentess/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,mtausig/RIOT,yogo1212/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,mtausig/RIOT,josephnoir/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT