Improving playlist path handling

[saratoga]

I've taken a quick step in this direction with:  http://gerrit.rockbox.org/r/#/c/270/

But our playlist handling is still not as painless as it should be.  Much of the problem comes from dealing with the ambiguity of mount points over MSC mode.  For instance,

/folder/file.mp3

Can refer can equally well refer to both:

/folder/file.mp3
or
/<microsd1>/folder/file.mp3

since we don't know where the root was for the software that made the playlist.  My patch above just moves the root to be whatever file system the playlist file is on, which is better then right now, but still not perfect. 

The next step would be to have the playlist code search multiple locations to resolve the ambiguity by looking at the file system.  As a start, we could search for files based on the root of each file system, and perhaps based on a few additional folders as the root of the file system (/music might make sense for instance). 

I'm not too familiar with this part of rockbox, but do people think this is practical?  I assume with dircache there would be basically no hit, and even without it usually the disk would be spun up anyway if someone has just clicked on a new playlist, but I'm not 100% sure.