Add a test for auth-pam searchandreplace

Add a test for auth-pam searchandreplace

No functional changes.

Utility functions of auth-pam are split into a dedicated file. This allows
the test programs to easily test these functions without adding
dependencies.

Add a minimal test for searchandreplace as a proof of concept.

[ Modified during commit: Enhanced documentation of functions in utils.h
  to comply with doxygen standards ]

Signed-off-by: Jens Neuhalfen <jens@neuhalfen.name>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20160525175756.56186-3-openvpn-devel@neuhalfen.name>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11724
Signed-off-by: David Sommerseth <dazo@privateinternetaccess.com>
(cherry picked from commit 4507bb6cd11799f72f1ede602315a60e03bb449c)