openvpn: update to 2.5.2

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>