Fix in-source builds

The internal header directory conflicted with in-source builds causing a
build failure. This is fixed by renaming the internal header directory to
something other than maxscale.

The renaming pointed out a few problems in a couple of source files that
appeared to include internal headers when the headers were in fact public
headers.

Fixed maxctrl in-source builds by making the copying of the sources
optional.

server/core/internal/admin.hh

@@ -0,0 +1,121 @@
+#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.
+ */
+
+#include <maxscale/cppdefs.hh>
+
+#include <string>
+#include <microhttpd.h>
+
+#include <maxscale/thread.h>
+
+class Client
+{
+    Client(const Client&);
+    Client& operator=(const Client&);
+
+public:
+
+    enum state
+    {
+        OK,
+        FAILED,
+        INIT,
+        CLOSED
+    };
+
+    /**
+     * @brief Create a new client
+     *
+     * @param connection The connection handle for this client
+     */
+    Client(MHD_Connection *connection):
+        m_connection(connection),
+        m_state(INIT)
+    {
+    }
+
+    ~Client()
+    {
+    }
+
+    /**
+     * @brief Process a client request
+     *
+     * This function can be called multiple times if a PUT/POST/PATCH
+     * uploads a large amount of data.
+     *
+     * @param url    Requested URL
+     * @param method Request method
+     * @param data   Pointer to request data
+     * @param size   Size of request data
+     *
+     * @return MHD_YES on success, MHD_NO on error
+     */
+    int process(std::string url, std::string method, const char* data, size_t *size);
+
+    /**
+     * @brief Authenticate the client
+     *
+     * @param connection The MHD connection object
+     * @param url        Requested URL
+     * @param method     The request method
+     *
+     * @return True if authentication was successful
+     */
+    bool auth(MHD_Connection* connection, const char* url, const char* method);
+
+    /**
+     * Get client state
+     *
+     * @return The client state
+     */
+    state get_state() const
+    {
+        return m_state;
+    }
+
+    /**
+     * Close the client connection
+     *
+     * All further requests will be rejected immediately
+     */
+    void close()
+    {
+        m_state = CLOSED;
+    }
+
+private:
+    MHD_Connection* m_connection; /**< Connection handle */
+    std::string     m_data;       /**< Uploaded data */
+    state           m_state;      /**< Client state */
+};
+
+/**
+ * @brief Start the administrative interface
+ *
+ * @return True if the interface was successfully started
+ */
+bool mxs_admin_init();
+
+/**
+ * @brief Shutdown the administrative interface
+ */
+void mxs_admin_shutdown();
+
+/**
+ * @brief Check if admin interface uses HTTPS protocol
+ *
+ * @return True if HTTPS is enabled
+ */
+bool mxs_admin_https_enabled();