dhcp: Clean up type handling of write_dhcp_*

dhcp: Clean up type handling of write_dhcp_*

Use more appropriate types. Add casts where
necessary but ensure that they are safe.

Change-Id: I30a50826350ac3176443cf3bf16d3972609723a2
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1268
Message-Id: <20251013161759.1656-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59246219/
Signed-off-by: Gert Doering <gert@greenie.muc.de>