Increase control channel packet size for faster handshakes

Increase control channel packet size for faster handshakes

Instead of limiting the control channel TCP/UDP packet payload size at
'100 bytes + real control channel overhead' (~140 bytes ethernet payload),
increase the max TCP/UDP payload size to '1250 bytes - calculated overhead'
(~1210 bytes ethernet payload).

Note that this patch does *not* yield an optimal solution, but it is a
simple and rather safe change that will improve connection setup times
significantly.

v2: use the mininum value of --link-mtu and 1250 to give the user a way to
    reduce control packet size if really needed.

trac #545

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1435693496-10931-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9841
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fc91d4b0071178e298052078431fb86f03be84fc)