OpenVPN update to 2.3.15 (security and bug fixes)

ChangeLog:
https://github.com/OpenVPN/openvpn/blob/v2.3.15/Changes.rst#version-2315

Details:
https://github.com/OpenVPN/openvpn/releases/tag/v2.3.15

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

This is not an MFH commit because the package has expired in head/ -
unfortunately it got technically expired only after the branch.
So we're in for another 7 weeks of maintenance...

Reported by: Samuli Seppänen
Security: 04cc7bd2-3686-11e7-aa64-080027ef73ec
Security: CVE-2017-7478
Security: CVE-2017-7479
Approved by: ports-secteam (feld@)