db/proxy: fall back to recursive walk on old libmpdclient/MPD

Error message was 'too few arguments for "find"' because the "base"
constraint was not supported, and no other constraints remained.

src/DatabaseSelection.hxx

@@ -44,6 +44,15 @@ struct DatabaseSelection {
 	DatabaseSelection(const char *_uri, bool _recursive,
 			  const SongFilter *_filter=nullptr);
 
+	gcc_pure
+	bool IsEmpty() const;
+
+	/**
+	 * Does this selection contain constraints other than "base"?
+	 */
+	gcc_pure
+	bool HasOtherThanBase() const;
+
 	gcc_pure
 	bool Match(const Song &song) const;
 };