main: replace how isodevice is found.
read it from /proc/mounts instead of wasting time with blkid.
block: load the value of the dos extended partition from the disk struct
block: initialize dosextended to DOS_EXTENDED
if DOS_EXTENDED2 or DOS_EXTENDED3 is found in
the partition table when loading from disk, then
set that as the value to use when handling
extended / logical partitions.