stand: Move dev_cleanup into libsa

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).

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38004