dns: Change arguments to setenv_dns_option to avoid sign-compare warning

dns: Change arguments to setenv_dns_option to avoid sign-compare warning

The change is a bit big to fix just one compare warning,
but that is due to the highly interdependent code.

Change-Id: Ibfcc350c772227cfc0f2244fa2b1625dcb7e6fb5
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1558
Message-Id: <20260407094643.28090-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg36531.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>