Fix checking for sender of rtnetlink messages
    
    So far we did check that incoming message comes has uid == 0. However this
    doesn't work if avahi is running inside net_ns && user_ns. Instead we should
    check that message comes from kernel, i.e. pid == 0.
    
    Thanks Lubomir Rintel <lkundrak@v3.sk> for the patch.
    
    Resolves: #1227052