dhcp: Replace DHCP Option types with defines

dhcp: Replace DHCP Option types with defines

Just nicer. Verified against
https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml

Change-Id: Ie41101bd00d038fa6fb906f3d30d44bf65788b96
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1266
Message-Id: <20251011082232.27602-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59245241/
Signed-off-by: Gert Doering <gert@greenie.muc.de>