Always disable TLS renegotiations

Always disable TLS renegotiations

Renegotiations have been troublesome in the past and also the recent
OpenSSL security problem (CVE-2021-3449) is only exploitable if
TLS renegotiation is enabled.

mbed TLS disables it by default and says in the documentation:

Warning: It is recommended to always disable renegotation unless you
know you need it and you know what you're doing. In the past, there
have been several issues associated with renegotiation or a poor
understanding of its properties.

TLS renegotiation can be used to restart a session with different
parameters (e.g. now with client certs). This something that OpenVPN does
not use.

For OpenSSL 1.0.2 the workaround to disable renegotiation is rather
cumbersome. So we keep this to 1.1.1 only since 1.0.2 is on its way to
deprecation anyway.

Furthermore because of all these problems, also TLS 1.3 completely
drops support for renegotiations.

Patch V2: Improve comments and commit message
Patch V3: Only disable renegotiation where the SSL_OP_NO_RENEGOTIATION
          define is available. LibreSSL, wolfSSL and OpenSSL 1.0.2 are
          lacking this macro.
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210401110003.19689-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21939.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 9e702a5d0f1d8ca0443d95ba13fc821deaa81d48)