Limit --reneg-bytes to 64MB when using small block ciphers

Limit --reneg-bytes to 64MB when using small block ciphers

Following the earlier warning about small block ciphers, now limit the
--reneg-bytes value when using a cipher that susceptible to SWEET32-like
attacks.  The 64 MB value has been selected with the researchers who
published the SWEET32 paper.

Note that this will not change a user-set --reneg-bytes value, to allow a
user to align a gun with his feet^w^w^w^w^w^w override this behaviour if
really needed.

v2: obey user-set --reneg-bytes 0 to revert to old behaviour, use more firm
    language in warning message, and add URL to man page.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1477655821-6711-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12798.html
Signed-off-by: David Sommerseth <davids@openvpn.net>