factor handler_logout
factor handler_logout

This commit (wrongly) assumes handlers will be passed a graph or a
device rather than a network. Surprisingly all tests are still passing,
which suggests that there are currently no tests verifying logout logic.
(I'm working on libmapper/libmapper#38 in case that wasn't obvious :)

radarsat1/libmapper,radarsat1/libmapper,malloch/libmapper,malloch/libmapper,radarsat1/libmapper,libmapper/libmapper,malloch/libmapper,malloch/libmapper,malloch/libmapper,malloch/libmapper,libmapper/libmapper,libmapper/libmapper,libmapper/libmapper,radarsat1/libmapper,libmapper/libmapper,libmapper/libmapper,radarsat1/libmapper