Update for changes in issue #279643, flvmediaplayer code upgrading.

- Simplify xspf_playlist_url() and enable custom base URLs to be passed
so the function can be called as needed from flvmediaplayer
- Update xspf_playlist_node_flvmediaplayer_file_use() to new arguments.
- Make $item_path arg optional in function
xspf_playlist_node_thumb_get() so that the function can be called for a
node per se rather than a playlist item.