Preserve ifconfig(_ipv6)_local across reconnect

Preserve ifconfig(_ipv6)_local across reconnect

Turns out that ifconfig_(ipv6)_local options are set once
and are not reset on a reconnect.

Consider following scenario:

 - connect first time, server pushes ifconfig-ipv6
 - add a ipv6 network route because ifconfig_ipv6_local is set
 - reconnect, server doesn't push ifconfig-ipv6

Because of ifconfig_ipv6_local is not reset and holds the value
set by the first connect, client adds a ipv6 network route -
but this is wrong, since ipv6 wasn't pushed this time by the server.

Fix by saving/restoring ifconfig(_ipv6)_local in a
struct options_pre_connect along with other options which
preserves the initial values until --pull modifications
are applied.

Github: OpenVPN/openvpn#850
Change-Id: I9b099924286f9bccb6833e1e40606abe72714bbb
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1248
Message-Id: <20251009171916.12811-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59244523/
Signed-off-by: Gert Doering <gert@greenie.muc.de>