make write_default_config optional

[cc32d9]

If an app starts without an existing config.ini, it creates a default config (if config-dir is not specified, the location is quite arbitrary, depending on current path).

appbase/application_base.cpp

Line 332 in fa081b4

write_default_config(my->_config_file_name);

It will be more practical not to write the default config, but exist with an error if the config does not exist.