fix single-session CD detection

fix single-session CD detection

ID_CDROM_MEDIA_SESSION_LAST_OFFSET is not set for CDs with only a single
session (i. e. for the vast majority of CDs out there). The previous rules ran
blkid with invalid arguments for these, causing CD detection to fail in
DK-disks and gvfs.

Now check whether we actually have ID_CDROM_MEDIA_SESSION_LAST_OFFSET, and if
not, call blkid without -O for specifying the offset.

Many thanks to Maxim Levitsky for tracking this down!

https://launchpad.net/bugs/431055