We introduce DBUG_UNREACHABLE macro to mark the places in the code
that must not happen.  We produce an error message stating that we
hit such a place in the code.

    * debug.[ch]:    DBUG_UNREACHABLE macro defined for both DBUG_OFF
                     and !DBUG_OFF cases.

                     exit_function was moved outside of !DBUG_OFF
                     region.

    * ctinfo.[ch]:   Removed CTIterminateCompilationSilient in favour
                     of existing CTIexit and replaced occurences.

    * main.c         Adjusted comments to match doxygen style, set
                     exit_function unconditionally to DBUG_OFF.