halt_error with a negative exit code did not exit with that code. The
code collided with the internal status values for null or false output
and for no output, so halt_error(-1) exited with 0, or with 1 under
--exit-status.
Keep the low 8 bits of a negative code, as exit(3) does on POSIX
systems, so halt_error(-1) now exits with 255 and halt_error(-2) with
254, the same with or without --exit-status. Under --exit-status this
replaces the previous absolute value, so halt_error(-2) used to exit
with 2 and halt_error(-255) with 255, and they now exit with 254 and 1.
