Allow tun-mtu to be pushed

Allow tun-mtu to be pushed

This allows tun-mtu to pushed but only up to the size of the preallocated
buffers. This is not a perfect solution but should allow most of the use
cases where the mtu is close enough to 1500 (or smaller).

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Patch v4: rebase for check_session_cipher name change
Patch v5: remove mention of change of default mtu, remove leftover code
          from an earlier approach.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221109154810.1268403-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25498.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>