Fixes two related security vulnerabilities (CVE-2020-15078) which
under very specific circumstances allow tricking a server using delayed
authentication (plugin or management) into returning a PUSH_REPLY before
the AUTH_FAILED message, which can possibly be used to gather
information about a VPN setup. In combination with "--auth-gen-token" or
a user-specific token auth solution it can be possible to get access to
a VPN with an otherwise-invalid account.

OpenVPN 2.5.2 also includes other bug fixes and improvements.

Add CI build test script.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>

New features:
* Per client tls-crypt keys
* ChaCha20-Poly1305 can be used to encrypt the data channel
* Routes are added/removed via Netlink instead of ifconfig/route
  (unless iproute2 support is enabled).
* VLAN support when using a TAP device

Significant changes:
* Server support can no longer be disabled.
* Crypto support can no longer be disabled, remove nossl variant.
* Blowfish (BF-CBC) is no longer implicitly the default cipher.
  OpenVPN peers prior to 2.4, or peers with data cipher negotiation
  disabled, will not be able to connect to a 2.5 peer unless
  option data_fallback_ciphers is set on the 2.5 peer and it contains a
  cipher supported by the client.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>