net/openvpn: Update to 2.6.7

Upstream NEWS:

Security Fixes:

* CVE-2023-46850 OpenVPN versions between 2.6.0 and 2.6.6 incorrectly use a send buffer after
it has been free()d in some circumstances, causing some free()d memory to be sent to the peer.
All configurations using TLS (e.g. not using --secret) are affected by this issue.
(found while tracking down CVE-2023-46849 / Github #400, #417)
* CVE-2023-46849 OpenVPN versions between 2.6.0 and 2.6.6 incorrectly restore --fragment configuration
in some circumstances, leading to a division by zero when --fragment is used. On platforms where
division by zero is fatal, this will cause an OpenVPN crash.(Github #400, #417).

User visible changes:

* DCO: warn if DATA_V1 packets are sent by the other side - this a hard incompatibility between
a 2.6.x client connecting to a 2.4.0-2.4.4 server, and the only fix is to use --disable-dco.
* Remove OpenSSL Engine method for loading a key. This had to be removed because the original author
did not agree to relicensing the code with the new linking exception added. This was a somewhat
obsolete feature anyway as it only worked with OpenSSL 1.x, which is end-of-support.
* add warning if p2p NCP client connects to a p2mp server - this is a combination that used to work
without cipher negotiation (pre 2.6 on both ends), but would fail in non-obvious ways with 2.6 to 2.6.
* add warning to --show-groups that not all supported groups are listed (this is
due the internal enumeration in OpenSSL being a bit weird, omitting X448 and X25519 curves).
* --dns: remove support for exclude-domains argument (this was a new 2.6 option,
with no backend support implemented yet on any platform, and it turns out that
no platform supported it at all - so remove option again)
* warn user if INFO control message too long, do not forward to management client
(safeguard against protocol-violating server implementations)

New features:

* DCO-WIN: get and log driver version (for easier debugging).
* print "peer temporary key details" in TLS handshake
* log OpenSSL errors on failure to set certificate, for example if the algorithms used
are in acceptable to OpenSSL (misleading message would be printed in cryptoapi / pkcs11 scenarios)
* add CMake build system for MinGW and MSVC builds
* remove old MSVC build system
* improve cmocka unit test building for Windows

(gdt)