Don't alert on valid ICMP6 solicit/advert messages.

Handles ND_ROUTER_SOLICIT, ND_ROUTER_ADVERT, ND_NEIGHBOUR_ADMIN,
ND_NEIGHBOUR_SOLICIT and ND_REDIRECT.  Don't set ICMPV6_UNKONWN_CODE
if code is the expected value of 0.

Buildbot output:
https://buildbot.suricata-ids.org/builders/jasonish/builds/53
https://buildbot.suricata-ids.org/builders/jasonish-pcap/builds/9