Rockbox

This patch addresses two issues concerning the current playing song in the Playlist viewer:

a)
After starting the Playlist viewer, the currently (last) playing song is not in the visible area:
Recipe to reproduce it:
1. the playlist should have more entries than the screen lines
2. play the last entry in the playlist
3. open the playlist viewer: the playing song is not visible (need to scroll 1 down)

b)(not a bug)
The currently playing song is always the last in the visible area:
1. the playlist should have more entries than the screen lines
2. play a song which position is greater than the number of lines on your screen
(e.g. if your display can show 10 lines then choose the 11th or above)
3. start the playlist viewer: the current playing song is the last on the list.
I'm not interested in the songs already played, but the songs that will come. Without the patch I always have to scroll down. With the patch the current song will show on the 2nd position in the list.

I adressed two problems I encountered in the playlist viewer, but the changes I made is in the file list.c. I don't think it has consequences on other lists.