Deprecate playlist_IsServicesDiscoveryLoaded()

With the introduction, initially for LUA SD, of the chain syntax to
the services discovery, there is no way to canonicalize the name of
a services discovery object. Thus, they cannot be compared reliably,
and therefore, playlist_IsServicesDiscoveryLoaded() does not operate
correctly. I won't remove it today because too many interfaces call it.
But you should know that this function does _not_ actually work, and I
see no way to fix it (other than remove LUA SD).

Futhermore, this function is fundamentally prone to a race condition,
when it is used to toggle an SD (this is admittedly a minor problem).

include/vlc_playlist.h

@@ -324,7 +324,7 @@ VLC_API int playlist_ServicesDiscoveryAdd(playlist_t *, const char *);
 /** Remove a services discovery module by name */
 VLC_API int playlist_ServicesDiscoveryRemove(playlist_t *, const char *);
 /** Check whether a given SD is loaded */
-VLC_API bool playlist_IsServicesDiscoveryLoaded( playlist_t *,const char *);
+VLC_API bool playlist_IsServicesDiscoveryLoaded( playlist_t *,const char *) VLC_DEPRECATED;
 /** Query a services discovery */
 VLC_API int playlist_ServicesDiscoveryControl( playlist_t *, const char *, int, ... );