More accurate seeking with libdvdnav

I'm experiencing a number of issues with search/seek/skip/rewind, see http://trac.xbmc.org/ticket/14493

Part of these issues is resolved by reviving a patch originally authored by 'gnosygnu' (see http://forum.videolan.org/viewtopic.php?f=32&t=76308&start=20#p316583) and submitted for inclusion by 'exobuzz' (see http://trac.xbmc.org/ticket/12212)

Full credit should go to gnosygnu, I just adapted the patch for the current codebase.

All DVD seek operations are noticably more accurate for me using this patch, and the "rewind jumps unexpectedly to the beginning" issue described as part of http://trac.xbmc.org/ticket/14493 is no longer reproducible with this patch applied.