MySQL modify user patch

This patch changed the function modifyUser(). Before this update
each data change was applied by deleting and re-adding the complete
user entry. The new function uses the UPDATE SQL statement.

Furthermore all human readable error messages were removed. The
calling procedure is in charge now to inform the user about
failures. Internal debug messages were added. They can be enabled
in the configuration file.

Last but not least the module retrieves the database version now
to handle incompatible features between different MySQL versions.

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