- reduced duplicate code in dns remoting function

- TODO: the server services plugin is not capable of enabling and disabling addon plugins, yet

Added remote functions config_value_get, config_value_add, config_value_update, config_value_replace and config_value_delete to access the sys_config key / value store in ISPConfig from remote api.

return xmpp servers

- fixed problem in remoting (not able to get version info without providing server_id argument), fixes #4801

and server_get_functions

This reverts commit 6e3c7edd.