OS X support for mount_vmware_shared_folder cap

- It's not clear whether it's possible to mount individual shares
  using 'mount -t vmhgfs', as the vagrant-vmware-fusion provider now does
  for Linux guests. Any mount from '.host', even if not a valid share
  name, succeeds and mounts _all_ shares in their respective directories
  at the root of the mountpoint. Instead, we symlink each directory from
  its place in '/Volumes/VMware Shared Folders'