Since dev_cleanup() walks through all the devsw devices with dv_cleanup
rotuines, move it into libsa rather than having it in
'common'. Logically, it operates only on things that are in libsa, and
would never be different for different loaders: either people would call
it as is, or they'd do the loop themselves with 'special' things inline
between calls to cleanup (not that I think that will ever be needed
though).

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