options: Introduce atoi_constrained and review usages of atoi_warn

options: Introduce atoi_constrained and review usages of atoi_warn

This is a more powerful version of atoi_warn that can
- check minimum and maximum values
- report error seperately from parsed value

This can be used to simplify a lot of option parsing.

Change-Id: Ibc7526d59c1de17a0f9d8ed88f75c6f070ab11e7
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20250902144657.11854-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59228172/
Signed-off-by: Gert Doering <gert@greenie.muc.de>