lib/strutils: improve strtoul_or_err() for negative numbers

lib/strutils: improve strtoul_or_err() for negative numbers

Let's use the same code for strtoul_or_err() and strtol_or_err() as we
already use for strtoxXX_or_err() functions. It resolves issue with
negative numbers.

This problem has been discovered by "./eject -x -1 -v" where -x is
based on strtoul_or_err(), but accepts negative number (-1).

Reported-by: Enze Li <lienze@kylinos.cn>
Signed-off-by: Karel Zak <kzak@redhat.com>