OpenVPN update to 2.4.2 (security fixes)

ChangeLog:
https://github.com/OpenVPN/openvpn/blob/v2.4.2/Changes.rst#version-242

Details:
https://github.com/OpenVPN/openvpn/releases/tag/v2.4.2

Security Announcement:
https://community.openvpn.net/openvpn/wiki/QuarkslabAndCryptographyEngineerAudits

Reported by: Samuli Seppänen
Security: 04cc7bd2-3686-11e7-aa64-080027ef73ec
Security: CVE-2017-7478
Security: CVE-2017-7479
MFH: 2017Q2