Support changing 9p mount tag at runtime

It would be helpful to be able to change the name of the 9p file share.
https://github.com/copy/v86/blob/4eb1e5035e91490c5aeebab45553d5043cd84b95/lib/9p.js#L82

For dynamically generating mount tags:
```javascript
/**
* @param {string} mountTagName
*/
function genMountTagArray(mountTagName) {
/**
* @type {Number[]}
*/
let _mountTagArray;
Array.from(mountTagName).forEach(function(char) {
_mountTagArray.push(char.charCodeAt(0));
});
return _mountTagArray;
};
```

Start at lib/9p.js around line 82, where the mount tag is defined, and trace how that value is used when the 9p share is mounted. Define the runtime behavior from the issue's requested mount-tag change and verify that a share can be renamed after initialization without breaking existing mounts.

Written by the indexing model from the issue text.