od: prefer signed types

od: prefer signed types

* src/od.c: Include stdckdint.h.
(bytes_to_oct_digits, bytes_to_signed_dec_digits)
(bytes_to_unsigned_dec_digits, bytes_to_hex_digits):
Use ‘char’ for these small constants.
(simple_strtoi): Rename from simple_strtoul.  Convert to int
instead of unsigned long; that’s good enough.  All uses changed.
Simplify by using ckd_mul and ckd_add to check for overflow.
(main): Prefer signed types to unsigned.