control/media_list.c: Revert the tagging of the subitems. This is not our job. Also not add the subitems to our list. We need a new media_tree object that will handle that itself.

 - specify server_port and, if needed, source
 - check for unsupported options
 - use proper default values
 - try transport successively until one works (untested!)

 - "handle" Require header
 - return 459/460 when appropriate

modules/misc/lua: Share some code between the modules that uses lua and put them in a single module.

Fixes #882

control/media_descriptor.c: Keep a media_list of our own subitems if we have some (we are then a media_descriptor playlist).

- Factorize some code

input/meta.c: Make sure we don't return VLC_SUCCESS in __input_FindArtInCache when no album art was found.