Add runtime enumeration of supported schemas.

Fix src/ls.cxx to only print unique schemas.
Refactor src/ls.cxx to use src/input/InputPlugin functionality.

Add dynamic enumeration support to curl plugin.

src/input/plugins/CdioParanoiaInputPlugin.cxx

@@ -357,4 +357,5 @@ const InputPlugin input_plugin_cdio_paranoia = {
 	input_cdio_init,
 	nullptr,
 	input_cdio_open,
+	nullptr
 };