use default MODEL unless explicitly set
Fixed 2.062 OS X regression introduced by 830e0770.

This reverts the module to the pre-830e0770 state for
non-Linux. It should probably be amended to follow the
same conventions as for other module files, which might
help to avoid such problems.
Merge pull request #479 from klickverbot/osx-statvfs

Fixed 2.062 OS X regression introduced by 830e0770.
Merge pull request #475 from dawgfoto/defaultMODEL

use default MODEL unless explicitly set