Change buffer allocation calculation and checks to be more static

Change buffer allocation calculation and checks to be more static

Currently we use half dynamic buffer sizes where we use have a fixed
overhead for crypto (crypto_max_overhead) but use a dynamic overhead
for the the other small header sizes.

Patch v3: rebase
Patch v4: add size of ack array to control channel frame size
Patch v5: fix calculation of compression overhead calculated over 0 instead
          of payload size

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220124025459.1042317-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23638.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>