Add a vmwarefusion driver flag to disable the mounting of the /Users
directory. Name it --vmwarefusion-no-share to reflect the pre-existing
virtualbox driver option --virtualbox-no-share.
Also add a corresponding environment variable FUSION_NO_SHARE.
The code is basically identical to the virtualbox driver's
implementation.
Signed-off-by: Joachim Viide <jviide@iki.fi>