Add helper functions to calculate header/payload sizes

Add helper functions to calculate header/payload sizes

These functions are intended to lay the groundwork to later replace
the distributed frame calculations and centralise the calculation in
one place.

Patch v2.2: clarify that the socks comments is assuming IPv4 and improve
            other comments

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211229163445.1893687-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23476.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>