util/grpcunix: Adjust to gRPC proposed authentication change
util/grpcunix: Adjust to gRPC proposed authentication change

This commit assumes a fork of gRPC with the changes described in
https://github.com/grpc/grpc-go/issues/111#issuecomment-83834867
Something like this should be enough to use the unofficial branch:

    go get -d google.golang.org/grpc
    cd "$(go list -f '{{.Dir}}' google.golang.org/grpc)"
    git checkout -b auth
    git pull https://github.com/bazillion/grpc-go auth
    go install

bazil/bazil,lisael/bazil