MXS-1446: Minor refactoring of gateway.cc

Cleaned up parts of the main function and moved the global configuration
initialization to the very beginning of the main function.

server/core/maxscale/config.h

@@ -42,6 +42,11 @@ extern const char *config_monitor_params[];
 extern const char *config_filter_params[];
 extern const char *config_server_params[];
 
+/**
+ * Set the defaults for the global configuration options
+ */
+void config_set_global_defaults();
+
 /**
  * @brief Generate default module parameters
  *