Fix broken DHCP options

Fix broken DHCP options

Commit

  2dfc4f ("dns: deal with --dhcp-options when --dns is active")

broke support for --dhcp-options. It removed the setting of the
DHCP_OPTIONS_DHCP_OPTIONAL flag for some DHCP options. This flag is
required for those options to be applied correctly, as it is used
when building the DHCP options string that is passed to the TAP driver.

This commit fixes the issue by restoring the setting of this flag.

GitHub: fixes OpenVPN/openvpn#791

Change-Id: I0d75efcceb826d06e74abd003d5377468ff9fe3b
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250730180527.25339-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32427.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>