WIP 'docker share' using rsync or sshfs - only does 'pwd'

Thought I'd just try it :)

The `--share=smb` share driver flag does not work - we need to merge #245 and some docker/pkg/mflags changes (or #243 and and some more fixes)
This could be _really_ fun if you did it from `/usr` >:)

Maybe we could have this check to make sure the directory you're trying to mount on top of either doesn't exist or is empty inside the VM before we mount? (potentially with a "force" flag)

Also, wouldn't it be useful to be able to specify both a "share source" and a "share target" here, optionally? (defaulting the first to `pwd` as it is below, and the second to the value of the first, of course, as it is now)