This commit adds a new protocol command to toggle protocol features

for a client connection. It works like the tag_mask and the associated
tagtypes command.

New commands:

- protocol
  Shows enabled protocol features.

- protocol available
  Show all available protocol features.

- protocol enable {feature...}
  Enables protocol features.

- protocol disable {feature...}
  Disables protocol features.

- protocol all
  Enables all available protocol features.

- protocol clear
  Disables all protocol features.

This commit adds also the first protocol feature.

hide_playlists_in_root
  Disables the listing of playlists in the root folder
  for the lsinfo command.

src/command/OtherCommands.cxx

@@ -160,7 +160,7 @@ handle_lsinfo_relative(Client &client, Response &r, const char *uri)
 	(void)client;
 #endif
 
-	if (isRootDirectory(uri)) {
+	if (!client.ProtocolFeatureEnabled(PF_HIDE_PLAYLISTS_IN_ROOT) && isRootDirectory(uri)) {
 		try {
 			print_spl_list(r, ListPlaylistFiles());
 		} catch (...) {