Decouple MSS fix calculation from frame calculation

Decouple MSS fix calculation from frame calculation

This consolidates the MSS fix calculation into a single function
instead having it distributed all over the code. It also calculates
the real wire overhead without extra sizes for buffer etc.

Patch v2: improve comment

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20211214150901.4118886-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23423.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>