Remove extra ] configure.ac for HAVE_DETECT_DISABLED

Saw a warning about unknow command ']' running distcheck and tracked it back
to this error.
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.