Everything in the disklabel is unsigned so make all the input routines
take unsigned values.
his allows one to label disk with the number of blocks > 31 bits
(though less then 32 bits)

e.g.

size offset fstype [fsize bsize bps/cpg]

c: 3125755904  0   unused    0     0        # (Cyl.    0 - 194569*)
d: 3125755840  64  unused    0     0        # (Cyl.    0*- 194569*)

which is needd to test UFS2