Fix --dns options for TAP adapter

Fix --dns options for TAP adapter

Commit

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

has accidentally removed setting of the DHCP_OPTIONS_DHCP_OPTIONAL
flag when copying --dns options. This flag is required to apply options
via DHCP string, which we do for TAP adapter. As a result, --dns options
stopped working for TAP.

Fix by setting this flag when copying --dns options to tuntap_options.

Change-Id: Id95cd14095a03afb3140a03ae96e9f5679e4fe89
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250731104833.23305-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32436.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>