Dan Williams authored Jul 23, 2015 and Thomas Haller committed Nov 10, 2015

Previously most objects were implicitly unexported when they were
destroyed, but since refcounts may make the object live longer than
intended, we should explicitly unexport them when they should no
longer be present on the bus.

This means we can assume that objects will always be un-exported
already when they are destroyed, *except* when quitting where most
objects will live until exit because NM leaves interfaces up and
running on quit.