Bug #429: Clean-up of reserved identifier violations

[Some identifiers](https://github.com/oetiker/rrdtool-1.x/issues/429 "questionable names") did not fit to the expected naming convention of the C/C++ language standard. This detail can be fixed by the deletion of underscores.
The probability for name clashes can also be reduced for the affected include guards by the addition of a kind of universally unique identifier as a suffix.