[RFC] Change the default for --inode-file-handles to prefer

Setting the default value of --inode-file-handles to prefer, I think I'll be safe, since it will fall back to FDs if file handles are not supported by the file system.

This is because users running virtiofsd as root sometimes hit the NOFILE limit, and increasing the limit using --rlimit-nofile, or using a different cache mode are available I think using file handlers is better.

Originally we discussed the default value here: #17 (closed)

I think keeping 1 to 1 compatibility with the C version is not an issue anymore, we already have some differences in other default values.

And since we are talking abut this, I also think we should use more consistent values, deprecating the current ones: prefer to auto, and mandatory to always