Allow DVD playback after 'browse into' ISO/IMG file (trac 14778)

see trac ticket 14778: http://trac.xbmc.org/ticket/14778 : DVD playback now works when browsing into an ISO file using our file manager.

The root cause was that libdvd (css/read/nav) did not recognize the udf://....iso path as the root 'folder', thus did not access the file via our VFS udf reader. 
Also, IFO files weren't read correctly due to a read position error in the code.
@elupus, could you review, as the UDF code was historically yours, and I want to make sure you agree with the changes. I noticed you weren't quite sure yourself as you left this comment 'why ?' in there...

I tried this PR on a number of DVD iso's each time using "Browse Into" and everything works well.