OpenVPN 2.7.6 (32-bit)

What's new in this version:

Security fixes:
- openvpnserv (windows): better scrutinize command line passed in from the control socket to openvpn. This would lead to circumventing admin restrictions on allowed openvpn config directories (but never to "read files the user has no permissions for")
- tracked in Github: OpenVPN/openvpn-private-issues#142)
- dco: make key state desync recoverable
- This was reported as a "with suitable timing, a key-update de-sync between OpenVPN and the kernel could trigger an ASSERT()", and was initially handled as security report. It turned out to be not exploitable, but the state machine was not very robust and so the opportunity was used to improve the code.
- make --x509-username-field work with mbedTLS.
- In very particular setups, together with a CA creating matching certificates, this could lead to unintentionally permitting a certificate that should not have. This is why this was considered a (low-prio) security bug and a CVE ID was assigned

Fixed:
- refuse incoming HARD RESET packets with a sequence ID != 0 (this is basically making an OpenVPN server ignore and log a
- "should never happen" client-side misbehaviour, which could lead to TLS handshake establishment failures in p2p TLS setups)
- correctly calculate packet id size if epoch packet format is in use - this was off by 4, for connections openvpn 2.7+ to openvpn 2.7+, exceeding "mssfix mtu" headroom by those 4 bytes
- correct minimum packet length check for 802.1q tagged packets (Github: #1044).
- This was also reported (twice) as a security bug, as technically OpenVPN with --client-nat would read and write up to 4 bytes "after the end of the packet" - but due to the OpenVPN packet buffer layouts, which are always full-frame-sized this is fully safe and has no adverse consequences.

User-visible Changes:
- if --dev is not specified, default to --dev tun - so for the tun case, this option can now be left out of the openvpn config
- --ping and --keepalive settings are now limited to 24 hours maximum - the primary reason for that is to avoid lots of extra code in the DCO kernel to handle arbitrarily large values without overflowing 32 bit integers. 24h is considered much higher than any reasonable use.
- The TCP_NODELAY socket flag is now "always on". The --tcp-nodelay option is kept, because setting it on a p2mp server also enables pushing of socket-flags TCP_NODELAY to clients, which might not have this code change yet.
- Remove --providers from --help output on mbedTLS builds

Building/Testing improvements:
- no longer use C99 hh scanf() length modifier, as it's not portable across all MinGW variants.
- fix test_tls_crypt test failures on Windows.
- t_client.sh: do not run resolvectl to query DNS settings if systemd is not running (= do not pollute log files with nonsensical errors)
- CMake builds: detect cmocka_version.h via include path, not by linking
- test_tls_crypt: Fix issue with temp file name on big endian systems (encountered trying to build 2.7.5 on Fedora s390x build instances)

Documentation improvements:
- ssl_pkt: Fix doxygen warning about read_control_auth