Fix datagram_overhead and assorted functions

Fix datagram_overhead and assorted functions

This function is supposed to calculate the overhead of the protocol
header (IP/IPv6 + TCP/UDP). But at some point the index that used
to index the array proto_overhead and the associated PROTO_N went
completely out of sync. This fixed the function and related caller
to again calculate the overhead as intended.

Note that IPv6 mapped IPv4 addresses still have the wrong overhead
calculated as they treated are as IPv6 addresses (0:0:0:0:0:ffff::/96)

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220101162532.2251835-5-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23504.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>