Prepare for 1.1.6-rc3.

Prep for 1.1.6 release, such as bumping version

Rework libnet_autobuild_ipv6() to eliminate code duplication.

Use libnet_build_ipv6() to build the packet rather than duplicating
it, and use new libnet_in6_is_error() function to check for
identity with in6addr_error.