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.

meson.build

@@ -379,6 +379,7 @@ sources = [
   'src/client/File.cxx',
   'src/client/Response.cxx',
   'src/client/ThreadBackgroundCommand.cxx',
+  'src/client/ProtocolFeature.cxx',
   'src/Listen.cxx',
   'src/LogInit.cxx',
   'src/ls.cxx',