Fix incorrect default mssfix value in server mode

Fix incorrect default mssfix value in server mode

When calculating default mssfix, we take into account
protocol overhead, which usually includes 3 bytes peer-id.

Peer-id usage is indicated by options->use_peer_id flag.
In client mode it is set when applying pushed options.
In server mode  it is not set and as a result mssfix value
is 3 bytes off.

Fix by setting this flag in multi.c when calculating
tunnel-specific options.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20220301135054.277-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23896.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>