libipvs: allow IPv4 RS in IPv6 VS and vice versa

This change syncronizes local copy of libipvs with the upstream (kernel/ipvsadm/ipvsadm.git) to the date.
IPVS in Linux 3.18 will include the feature of mixing of tunneled RS families in single VS by Alex Gartrell.

The compatibility with older kernel versions is kept.
libipvs: minor bugfix with retreiving dest af

This change needs to be sent to the ipvsadm upstream, too.
This clarifies the previous commit, so there is no need to mention it in the changelog.