Remove the unix file before listening if the server is running as unix server

[hwchiu]

No description provided.

[John-Lin]

We should unlink before bind and follow UNIX principle. see simple UDS server example in C.

https://github.com/troydhanson/network/blob/master/unixdomain/01.basic/srv.c#L28-L29

https://troydhanson.github.io/network/Unix_domain_sockets.html#unlink-before-bind

err = syscall.Unlink(socketpath)
if err != nil {
    // not really important if it fails
    log.Error("Unlink()",err)
}

log.Println("Starting gRPC server")
ln, err := net.Listen("unix", "/tmp/go.sock")