Move build_dhcp_options_string from tun to dhcp

Move build_dhcp_options_string from tun to dhcp

Seems suitably related and tun.c is one of the
huge ones.

In preparation of adding UTs for the code.

Change-Id: I62a8f62b9c0938cfcb99d184b07034515c076303
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1265
Message-Id: <20251011082217.27568-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59245240/
Signed-off-by: Gert Doering <gert@greenie.muc.de>