Rework iPod scrobbling on Linux / FreeBSD

Change device handling to be more like win/mac
 - each device has its own playcounts.db
 - use the same table structure as the mac db
 - devices are no longer associated with users; they all go to the
   current user (like in 71119eb2)
 - listen to D-Bus and try to scrobble an iPod once it's mounted
 - if there's no useful D-Bus interface, fall back to manual iPod
   scrobbling

Difference from win/mac: The Linux client only scrobbles the Recent
Playcount for tracks and then resets the recent playcount on
completion.  This is needed because other programs like Amarok can
also scrobble iPods, and we want to avoid significant over-scrobbles
Enable linux ipod scrobbling through the diagnostics dialog
Remove some cruft