Make the buffer low-level watermark (and the related anti-skip setting) work again...

Make the buffer low-level watermark (and the related anti-skip setting) work again. Update the buffering 'base handle' as new tracks start playing, since this is needed to determine how much playable data is left in the buffer. Should fix the buffering problem reported in the forums (forums.rockbox.org/index.php?topic=24014.0), and also makes the usefl line in the buffering thread debug screen useful again.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@24968 a1c6a512-1295-4272-9138-f99709370657