multi: store multi_context address inside top instance

multi: store multi_context address inside top instance

Future modifications to DCO require accessing the
server multi_context object.

Since it is currently a stack variable that is pointed
by no one, we'd need to pass it to all kind of functions
to ensure it can reach the DCO code.

To make the implementation simpler, it is preferable to
simply assign its address to a struct context's field.

While at it, make some multi_* functions static as they
used only inside multi.c, where they are defined.

Change-Id: Ibf64c681e02ac572d339d4d98e75ceb0cd417c45
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250723061034.20240-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32266.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>