disk/part_dos: check harder for partition table

Devices that used to have a whole disk FAT filesystem but got then
partitioned will most likely still have a FAT or FAT32 signature
in the first sector as this sector does not get overwritten by
a partitioning tool (otherwise the tool would risk to kill the mbr).

The current partition search algorithm will erronously detects such
a device as a raw FAT device.

Instead of looking for the FAT or FAT32 signatures immediately we
use the same algorithm as used by the Linux kernel and first check
for a valid boot indicator flag on each of the 4 partitions.
If the value of this flag is invalid for the first entry we then
do the raw partition check.
If the flag for any higher partition is wrong we assume the device
is neiter a MBR nor PBR device.

Signed-off-by: Egbert Eich <eich@suse.com>