sout: allow module-less sout creation

By exposing a new close callback in the sout v-table it becomes possible to create sout objects not tied with modules. It allows to programmatically create custom stream outputs without having to declare them as modules usable by the user.

This will be used in tests, to replace the current chromecast-proxy implementation and to forward PCR properly in duplicate (we need specific sout filters here to catch PCR at the end of each duplicated stream).