add database-size monitoring. the results stored in the database may be used to add the db-size to the web-quota, too.

- Added "actions" framework in server to replace the functions provided by the core modules
- Moved system update function from remoteactins core module to software update plugin.

and make changes to mysql users in the mysql plugin module.

Refs: 1722

Also removed the calls to the db closeConn calls in favor of
destryong the connections on the class __destruct() 's..

Since firebird db had a closeConn as well, changed its __destruct
to calls its own personal closeConn which otherwise left unmodified.

Refs: 1722

- replace double-quotes with single-quotes whenever appropriate
- fix indentation

- reword some output
- fix typos

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Fixed general utf8 encoding problems when data is passed from frontend to server, this fixes also bug FS#1065 - Special characters in autoresponder subject may break the autoresponder 

Added a missing language string in server config form.

- Updated Debian installation instructions.