openvpn: update to 2.6.5 and add DCO support

openvpn: update to 2.6.5 and add DCO support

This commit updates openvpn to version 2.6.5 and add DCO support.

There are several changes:

- Starting with version 2.6.0, the sources are only provided as .tar.gz
  file.

- removed OPENVPN_<variant>_ENABLE_MULTIHOME:
  multihome support is always included and cannot be disabled anymore
  with 2.6.x.

- removed OPENVPN_<variant>_ENABLE_DEF_AUTH:
  deferred auth support is always included and cannot be disabled
  anymore with 2.6.x.

- removed OPENVPN_<variant>_ENABLE_PF:
  PF (packet filtering) support was removed in 2.6.x.

- The internal lz4 library was removed in 2.6.x; we now use the liblz4
  package if needed

- To increase reproducibility, _DATE_ is only used for development
  builds and not in release builds in 2.6.x.

- wolfSSL support was integrated into upstream openvpn

- DES support was removed from openvpn

The first two wolfSSL patches were created following these 2 commits:
https://github.com/OpenVPN/openvpn/commit/4cf01c8e4381403998341aa32f79f4bf24c7ccb1
https://github.com/OpenVPN/openvpn/commit/028b501734b4a57dc53edb8b11a4b370f5b99e38

Signed-off-by: Martin Schiller <ms@dev.tdt.de>