Add unit testing support via cmocka

Add unit testing support via cmocka

cmocka [1,2] is a testing framework for C. Adding unit test
capabilities to the openvpn repository will greatly ease the
task of writing correct code.

cmocka source code is added as git submodule in ./vendor. A
submodule approach has been chosen over a classical library
dependency because libcmocka is not available, or only
available in very old versions (e.g. on Ubuntu).

cmocka is build during 'make check' and installed in vendor/dist/.

[1] https://cmocka.org/
[2] https://lwn.net/Articles/558106/

Signed-off-by: Jens Neuhalfen <jens@neuhalfen.name>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20160525175756.56186-2-openvpn-devel@neuhalfen.name>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11725
Signed-off-by: David Sommerseth <dazo@privateinternetaccess.com>
(cherry picked from commit 40cb4cfc5d011102daec61ab39583cba0eeb3077)