command: add command "listfiles"

Lists files and directories.  Supports storage plugins.

src/command/DatabaseCommands.hxx

@@ -24,6 +24,9 @@
 
 class Client;
 
+CommandResult
+handle_listfiles_db(Client &client, const char *uri);
+
 CommandResult
 handle_lsinfo2(Client &client, int argc, char *argv[]);