Add first version of PAM authenticator plugin

This includes the client and backend authenticators. Currently,
only a simple password-based scheme with the SQL-client "dialog" plugin
is supported. In this mode, the server sends the first PAM message
with the AuthSwitchRequest packet and the client responds with the
password. No further authentication messages are supported. If the
connection is not encrypted, the password is sent in plaintext. The
client password is used as is for logging in to backends.

server/modules/authenticator/PAM/pam_auth_common.cc

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include "pam_auth.hh"
+
+#include <maxscale/alloc.h>
+#include <maxscale/log_manager.h>