Implement optional mtu parameter for mssfix

Implement optional mtu parameter for mssfix

The current mssfix parameter is a bit difficult to use as it needs
manual calculation of the allowable packet size and also the resulting
MSS value does not take into account if IPv4 or IPv6 is used on the
outer tunnel.  Add 'mtu' parameter to fix both of these problem by
dynamically including the real overhead.

The syntax and naming of the parameter is chosen for compatiblity with
OpenVPN3.

Patch V2: document mssfix 0 disabling mssfix, fix rst syntax

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220101162532.2251835-6-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23495.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>