Fix reserved identifier violations

This is a companion update suggestion for the issue "[reserved identifier violation](https://github.com/cockpit-project/cockpit/issues/201 "Check standard-compliance of identifiers")".
* Include guards prevent the multiple inclusion of header files only correctly if the chosen identifiers are different from others with such a functionality. The probability for name clashes can be reduced by the addition of a kind of universally unique identifier as a suffix.
* Affected data structure identifiers can become standard-compliant by the deletion of leading underscores.