Extract multi_assign_peer_id into its own function

Extract multi_assign_peer_id into its own function

This makes multi_get_create_instance_udp a bit shorter and better
structured and also prepares this method to be called from the
mutlti TCP context with DCO which will also need to assign unique peer
ids to instances.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210401131337.3684-13-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21959.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>