Fix fdisk(8) to create 2TB partitions on disks larger than 2TB, rather than
only being able to create 1TB partitions:
o) Use an unsigned 32-bit quantity to store the number of disk sectors.
o) Detect overflow of said 32-bit quantity and clamp to 2^32.
o) Rather than returning the disk sector count from get_params, return 0 on
   success, since its return value is only ever compared to -1 to detect
   failure.  This would cause returning 2^32 sectors to be interpreted as an
   error.

Reviewed by:	bde ("good for a quick fix")