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!
Right, but if the "share directory" is empty, we don't even try sharing.
 So with this, the experience on Linux goes from "boot2docker
--vbox-share-dir=/home --vbox-share-name=Users" to the much more verbose
"boot2docker --vbox-share=true --vbox-share-dir=/home
--vbox-share-name=Users", when there isn't really a great reason for it
that I can see for the inconsistency, unless I'm missing something.

Otherwise, I'm +1 on this change and totally agree that it helps with
increasing visibility on the default behavior immensely.