PAM code cleanup & refactor

Divided functionality into classes, fixed comments +
various other cleanup. BackenAuth no longer increments
sequence on sending password. SQLite busy timeout shortened
to 1 second.

server/modules/authenticator/PAM/PAMAuth/pam_auth.hh

@@ -0,0 +1,27 @@
+#pragma once
+/*
+ * 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.
+ */
+
+/*
+ * Common definitions and includes for PAM client authenticator
+ */
+#define MXS_MODULE_NAME "PAMAuth"
+
+#include <maxscale/cppdefs.hh>
+
+#include <maxscale/alloc.h>
+#include <maxscale/buffer.hh>
+#include <maxscale/dcb.h>
+#include <maxscale/protocol/mysql.h>
+
+