Let count=-something fail, while count=0 may succeed, thus making dd(1)

useful as a seeking-tool as well as its many other uses.  Previously,
dd(1) would succeed with count=0, but wouldn't get to the point that
blocks were to be read/written. This is a more useful behavior, and
this specific case doesn't seem to be handled by POSIX.

bin/dd/args.c

@@ -207,8 +207,8 @@ f_count(arg)
 {
 
 	cpy_cnt = get_num(arg);
-	if (!cpy_cnt)
-		terminate(0);
+	if (cpy_cnt < 0)
+		errx(1, "count cannot be negative");
 }
 
 static void