Make a registry of modules
Assign an unique number to each module with the MODULE_FLAG bit set
so that we can also enumerate them
Implement enumerating our internally loaded modules
Implement unloading of the module using the module id
Move module-null-sink into a separate file
Use Audio/Sink if no other media.class was given, so that it works
just like the pulseaudio module
Enable linger=true in all cases.