Exit with the given status for negative halt_error codes (#3660)

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.

tests/shtest

@@ -471,6 +471,15 @@ elif [ $? -ne 11 ]; then
     echo "jq halt_error(11) had wrong error code" 1>&2
     exit 1
 fi
+for opt in '' -e; do
+    if $VALGRIND $Q $VG_EXIT0 $JQ $opt -n 'halt_error(-1)'; then
+        echo "jq $opt halt_error(-1) didn't work as expected" 1>&2
+        exit 1
+    elif [ $? -ne 255 ]; then
+        echo "jq $opt halt_error(-1) had wrong error code" 1>&2
+        exit 1
+    fi
+done
 if [ -n "$($VALGRIND $Q $JQ -n 'halt_error(1)' 2>&1)" ]; then
     echo "jq halt_error(1) had unexpected output" 1>&2
     exit 1