Werk #1875: Fixed possible exceptions of CMC Check_MK helpers when using some custom checks

When using the CMC together with some custom checks which register variables in the global context which can not be written out with repr() and read with eval() like for example python module references or function pointers, all Check_MK workers stopped working and were restarting in an endless loop.

A verification has been added to prevent those variable from being written to the Check_MK check helper config var/check_mk/core/config.mk.

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