playlist: Correctly indicate why we put a buggy yield() at this place. And document a bit that nasty recusrsive ML_Decref() function.

This comes with my official excuses to funman.

Please TEST your changes before pushing
At least, READ the code instead of modifying it blindly

Pointed-out-by: Rafaël Carré <funman@videolan.org>

playlist: To avoid race on p_input destruction, keep a reference around while we need it in playlist_MLLoad().

playlist: Proper locking when editing playlist variable in loadsave.c. Also correctly release p_ml_onelevel->p_input and p_ml_category->p_input when changing them.

This fixes partly make check and input items leaks.

There is no need to copy the files in that case thanks to VPATH.
That assumes you are using a source tarball then - if not, you are
screwed anywhow.

This reverts commit 70f620e7f94eb02fc6f5eda47599ae5354c12483.

Fix make distcheck
(cherry picked from commit 70f620e7f94eb02fc6f5eda47599ae5354c12483)

(needs to check make distclean now)