move the defaults to where the flags are defined so the user knows what ...

...the values look like, and report at least one error

merge me!
thats a very EW way to quietly toggle the boolean.

its horribly inconsistent to say 
- `--vbox-share=true --vbox-share-dir=""` - imo that should be an error condition, not a silent change to the bool.

essentially, if you want to toggle based on the `dir` string, then don't provide an enable bool.

I prefer the bool to mean what it says.