pulse-server: improve module handling

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.

src/modules/module-protocol-pulse/defs.h

@@ -54,6 +54,7 @@
 #define INDEX_MASK		0xffffu
 #define MONITOR_FLAG		(1u << 16)
 #define EXTENSION_FLAG		(1u << 17)
+#define MODULE_FLAG		(1u << 18)
 
 #define DEFAULT_SINK		"@DEFAULT_SINK@"
 #define DEFAULT_SOURCE		"@DEFAULT_SOURCE@"