build: avoid -Wstrict-overflow warnings with GCC 5.1 on 32 bit

build: avoid -Wstrict-overflow warnings with GCC 5.1 on 32 bit

* src/shred.c (dopass): With -O2, GCC 5.1 gives "assuming signed
overflow does not occur when simplifying conditional to constant"
warnings, in regard to the signed (off_t) variables.  Therefore
use unsigned (uintmax_t) instead, and a separate boolean to cater
for the special meaning of the negative part of the integer range.
Noticed at http://hydra.nixos.org/build/24983447