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/pam_auth_common.hh

@@ -0,0 +1,43 @@
+#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 declarations for both PAMAuth and PAMBackendAuth
+ */
+#include <maxscale/cppdefs.hh>
+#include <string>
+
+extern const std::string DIALOG;
+extern const std::string PASSWORD;
+extern const int DIALOG_SIZE;
+extern const char GENERAL_ERRMSG[];
+
+/** PAM authentication states */
+enum pam_auth_state
+{
+    PAM_AUTH_INIT = 0,
+    PAM_AUTH_DATA_SENT,
+    PAM_AUTH_OK,
+    PAM_AUTH_FAILED
+};
+
+/* Magic numbers from server source
+   https://github.com/MariaDB/server/blob/10.2/plugin/auth_pam/auth_pam.c */
+enum dialog_plugin_msg_types
+{
+    DIALOG_ECHO_ENABLED = 2,
+    DIALOG_ECHO_DISABLED = 4
+};
+