shell/gnrc_icmpv6_echo: don't do DNS lookup for plain IP
shell/gnrc_icmpv6_echo: don't do DNS lookup for plain IP

When using ping6 with an IP address, don't do a DNS lookup.
Hostnames can't contain ':', so use that to tell them apart
from plain IP addressees.

OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,smlng/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,smlng/RIOT,ant9000/RIOT,ant9000/RIOT,basilfx/RIOT,jasonatran/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,smlng/RIOT,RIOT-OS/RIOT,miri64/RIOT,yogo1212/RIOT,basilfx/RIOT,miri64/RIOT,miri64/RIOT,smlng/RIOT,authmillenon/RIOT,basilfx/RIOT,kaspar030/RIOT,yogo1212/RIOT,kYc0o/RIOT,kYc0o/RIOT,yogo1212/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,basilfx/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,basilfx/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,authmillenon/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT,yogo1212/RIOT,kYc0o/RIOT,kaspar030/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,smlng/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT